My Fuji Hybrid has a three ring crankset for the 3 x 7 gearing. I recently changed the big ring from a 48 to 52 and with a 28 small ring leaves a 24 tooth difference. The stock derailleur is a Shimano Tourney that handles a recommended 20 tooth difference. The derailleur does work but I'm interested in upgrading the unit using the Shimano components. I've also changed to Shimano ST EF51 click shifters from the original twist grip that seem to work well.
Please advise on a front derailleur upgrade.

If it works........
What problems are you having?

Everything on a bicycle works together. You have stretched the design capacity of the derailleur and have no trim function on your shifters. The question now is: "Does it work well enough to meet your standards?" If your answer is "No." my advice would be to switch shifters to something that provides trim capabillity rather than swapping front derailleurs.

Shimano tends to be pretty conservative when it comes to published limits and capacities. I'm not shocked that you could get a few extra teeth in there. (On the other hand, I wouldn't be shocked if you get a little chain rub in certain gear combinations because you're pushing the limits, either.)

If your current setup is working, what are you looking to gain from a derailleur upgrade?

Another question-
What would you up"upgrade" to?
Most of the "upgrade" FD's are for more speeds, thus will have a narrower cage, thus will be fussier to get adjusted properly without chain rub.
You are more likely to induce new problems.

This is a Tourney on my 3X9 with 22-32-36 rings.
It was a PAIN to get adjusted to work well, but it works. I went through an extra cable, because of all the adjusting (clamp, unclamp, clamp...)

IF it works OK, count your blessings.

What do you hope to accomplish with such an upgrade? If you have a front derailleur which shifts satisfactorily there is little else to wish for.
"Never Fix A Running Piece" would seem to apply here.

I don't have anything clever to say about upgrades, but I just spent an 'exciting' thirty minutes adjusting the Altus front derailer on my new-ish Trek Allant 7.2 (same as FX 7.2, but with disc brakes). I had an odd noise on the middle chainring, almost as if there was a ticking freewheel on the crank axle. The chain was not rubbing, and the noise didn't occur on the small or big chainring. I couldn't figure it out for a long time and kept adjusting the cable tension until the shifting was perfect and rubbing absolutely minimal. Still a weird noise on the middle chainring and middle cog, right at the best chain line. I thought maybe this is the best you can get with these cheap derailers.

Then I finally got my ears and eyes close enough to the derailer and realized that there was a partly clear plastic sticker hanging from the derailer cage and it rubbed on the chain when the derailer was positioned just so, on the middle chainring. The sticker was apparently part of Shimano's packaging/installation instruction, and would have been ok if it hadn't torn halfway off. Took out the sticker and the noise was no more.

Moral of the story: whatever you do, remove Shimano's stickers.

I find the shifting actually good enough, once you get the cable tension right. I had trim on a previous bike (Shimano bar-ends), but I don't really miss it. You can't avoid rubbing entirely on the Altus shifters, but I can get rid of it on the gears I actually use.

Actually, the EF-51's do.
My advice to leave well enough alone still holds.

Really how?

Because it was good advice.

Hah! I meant, how does it do trimming. I have this shifters and never knew they did trimming.

Probably a bit of confusion in my translation.
I meant it has cable adjustment, which is what I initially thought was inferred.