I've modded my XTC too. I have a custom faceplate being made for it and the SL+. I'm not a pro with amp building or anything, I just like to try stuff. I've practiced soldering enough now that I'm confident enough to mod my modules.
As far as the SL+ goes, I've upgraded all four of the large caps in the middle area of the module. They're still the same value's, they just sound better. I had done the 800 mod previously as mine was from pre 800 mod era, but I tried changing R25 to a 39k resistor and it made the mids more Slashy I thought. From what I hear it will change the frequency of the mids without affecting anything else. I'm still planning to upgrade the bass cap and possibly change the value too, and I'll try replacing some of the 100k resistors with carbon comp resistors. I'm open to other changes too. But I don't mind the sound already.