Since I didn't get an answer fast enough and I got FFT yesterday, I hunted down to info on my own, but here's what I found, for other people that might need the same.

Here's the basic : going from 1.5 unmodified firmware to the new M33 3.71.

First steps are getting your system to m33 3.52, as 3.71 won't install straight from it. This website had a very easy to use guide (althought some of it are a tad confusing) :
Stylemo : How to install 3.52 M33-4 on your PSP

The main difference, not clearly written, is that you actually stop at step 3 folder (he wrote step 5 somewhere in the instructions), and that you need to manually create a GAME150 folder on step 3 to put the update file in. Otherwise, easy as pie.

Stylemo also has an installing guide for M3.71, but the file he wants you to use is missing, so I had to do it on my own. This time, with a m33 3.52-4 installed, it was much easier to figure out.

I got the 3.71-1 update here :
PSPUpdates M33 3.71-1

For which you need the 3.71 Eboot :
PSP 3.71 Update

And then 3.71-2 update here, for which you don't need the 3.71 eboot anymore :
PSPUpdates M33 3.71-2

FFT works perfectly

Don't know if it was written anywhere else on this site, but if this helps anyone, so be it.

 

For the simple task of running FFT and FF1 PSP versions? None yet. I haven't fully messed with my PSP like going online with it yet, I have enough computers that net browsing is kinda redundant on it, not to mention small as heck.

So if you really just want to play commercial games without losing your homebrew ability, M33 seems to work fine. It looks like an official 3.71 for the most part. And if you need to run stuff off the 1.5 Kernel there's an add-on that allows it. From what I can see, M33 gives you all the bells and whistles that newer games require you to update for, without losing the 'open windows' in the code needed to run homebrew.