Bicycle Mechanics - Front derailleur question

I know this is probably a dumb question, but I'm going to ask anyway. What will happen if I try to use a front derailleur (FD-M581) that Shimano specifies as having a "min. difference between top and intermediate" of 12T with a 48/38/28 crankset?

Don't do it. Mismatched chainrings and front derailleurs are the leading cause of getting your scrotum caught in the chain.

Yes, that was not helpful.

I tried Google before posting here and didn't find anything, but with a little more perseverance afterward I did find my answer. For the curious, the bottom of the derailleur cage rubs the middle ring when the big ring is in use (at least potentially).

An interesting artifact of this seems to be that you can't use anything above an Alivio-level derailleur on a crankset with 48-38-28 rings (which, of course, is probably not above Alivio level itself), and on the road side of things, you apparently can't use anything above a 105-level triple front derailleur on a 50-39-30 crankset (which is also something I was contemplating -- Ultegra 6603 FD with my FSA Gossamer triple).

You could probably raise the DER up enough to clear the middle ring, but your shifting would "technically" be compromised.
Sometimes they'll still work fine though.

I wonder if you found my thread about this. I was trying to use a 6603 front derailler with a 52/42/30 crankset. The 10 tooth difference between the big and middle rings meant that I had to have the FD a good 6mm above the big ring to clear the middle ring due to the deep inner plate of the 6603 FD. My original solution was to use a 105 9 speed FD but this past year I swapped that for a 105 10 speed FD which shifts much nicer with the 10 speed chain. I still need to have the FD set at the max point above the big ring (~3mm) to just barely clear the middle ring but it works great so I'm not complaining.

Wow Deja Vu I have been looking for something on this with the exact same triple. The reason is I bought a new wheelset and opted to get a new SRAM cassette and I am amazed at how well and quiet it shifts. My FD is a little lazy and noisy so I was wondering if I do new complete crank or a Dura Ace derailler??

Rule of thumb:

If it's a part that I already own or that I can get for free I will install it on the bike and see if I can make it work to my satisfaction. I don't charge myself labor so there's no money lost and often I learn something useful in the process.

If I'm buying a new part, or even one that's been pre-broken-in, I'll hold out for one that matches.