My understanding is that this deraileur is designed for 13 tooth difference middle-large ring, and for 10sp drivetrain. My experience with Shimano deraileurs, front and rear, is that they typically offer good performance beyond stated range, and am thinking this should work with 53-39-30, which would be 14 tooth, with 105 5700 shifters. Any experience with this setup?

Closest experience I have is 52/40/26 with a 5500 FD and shifter. the 26 to 40 shift is a little slower than it was with 30-40. I've just learned to hold that shift a bit 'til it shifts up.

Our experience with Shimano 105 52/42/30 converted to 53/39/24 resulted in no issues with a 12-27 9sp. Tandems IMO *should* have additional high (and low) end over 1/2 bikes.

My experience with a converted tandem with a 53-42-30 crankset originally in 9 speed with a 5603 derailleur converted to 10 speed with 6603 derailleur and shifter it does not work well at all, the FD 6603 has a big cage and hit the 42 ring when trying to shift the big ring, have to move the FD too high and makes for poor shifting. But its not the case for yours since you have a 39t middle ring, but I changed the rings to 52-39-30 and still not working well enough.

I will try the recommended derailleur from people here that suggested the CX-70 frond derailleur with a small and smooth cage. hope it will work better.

Eddie 50 is correct, older FD for 52/42/30 are different from 52/39/30 with the 39t FD having a larger vertical height in the back side of the FD
cage, this larger vertical height hits the 42t CW when upshifting from 42 to 52. You can see the differences in cage height on the FD
on this page: Front Derailleurs

So going from a stock 39t mid CW to 42t mid will result in FD incompatibilities.

There are some very interesting chain catchers that bolt on to FD with integral FD frame mounts, they use a longer FD mounting bolt
and mount in front of the FD on the integral mount and reach down to prevent the chain from falling off the small CW when down
shifting, but they work with clamp on "braze-on" adapters as well if you have a seat tube compatible with this.

Chain rings aside......we have had 2 of the 6703 left shifters fail. Both failed in the same manner. Sticky shifting for a half dozen times and then just nothing. The first time, we were stuck in the big ring,so I cut the cable to get home. This time (last week) we were in the small ring so we just pedaled on home and did a bit of coasting. We are on vacation, so I took the bar end shifter that usually runs my third brake and repurposed it for the front D/R. I really like it. And I can always tell which chain ring we're in.

Before I get flamed, two different bike shops looked at the system. It's not the cables and its not friction or the bike or anything else. A lot of people think the 6703 is a crappy shifter, and I agree. We had it on our tandem for three years, new from the factory, custom built. It always shifted like crap and was hard to adjust. If not set just right it would skip right over the middle ring on the down shift, hit the granny gear. Or you'd have trouble getting it on to the big ring. Or it would overshift (did that once which was real exciting).

Maybe it works better on single bike with shorter cable runs. I don't know. But $500 worth of trashed shifters is enough for me.

As a rule I am done with triples on single bikes. Totally avoidable with the right chainring combo. But on tandem I am not quite there yet. Nice field repair/fix using the bar end shifter. One could also fix the fd in the middle ring if the shifter fails, whether stuck in big or small. Not sure about 6703 shifter. I have mostly 7700, 7703, and 5703. With the exceptiion of the newish 5703, all have been fine over many tens of thousands of miles. The one shifter I have had fail was in fact an Ultegra right shifter. I think it was 6500 or 6600 though.

I did consider going with bar end for fd. I have simplex retrofriction I could use. (Incidentally, I had two of these fail-both rear shifters-with springs breaking and/or cylinder freezing up. my fix was to fix the rear derailer in a middle cog.)

At this point I think I am getting the 6603 to work with 53-39-30. Functionality is much improved bending the plates a little closer together. The tradeoff may be needing to trim when in large or small cog. The height of the inner plate seems well-matched for this combo. In fact, I can position the derailleur lower than with the 52-39 without the inner plate fouling the middle ring as sch mentions. All the contours on the plates line up with the chain properly when in the various rings.

I had the 6703 shifter go out on us on our trip to the San Juan Islands last summer. The local shop could not fix it so I ordered a new one with next day shipping to save the vacation. When I got home and did some research, found this post shimano st 6703 problem... - BikeRadar Forum that matched my problem. There is a small specialized nut that holds it all together that works loose. Mine was lost. I was able to use a nut from an old erector set to get the shifter working again.

Well yeah, FDs designed for 39T don't work well with 42T. Simple solution: replace the middle ring with 39T. Then it works perfectly. Going the other way is not an issue: FD's designed for 42T work perfectly with 39T. Going to a 26T instead of a 30T works well, too. Some folks run 24T but not all FD's will allow that small a ring and I don't know which will and won't.

But that's not the OP's question, which is about going to a 53. Yes, a 53T should work fine, though you may need to add a link for big-big. OTOH, maybe the question is "should I replace my 5700 shifters with 6300 in order to run a 39T?" To which I beleive the answer is "unnecessary."