Sorry I am a bit late to the game - I have been out of airsoft for a while. How is the upgrading going?
I don't have the A1 but I have two XB's. One of them needed a complete overhaul so I ended up making/replacing a lot of parts. The other I am doing up to sell it - I am remaking the hop unit to be VSR compatible and making the trigger and sear from steel because the stock ones have worn out. The stock cylinder set is pretty good but I wouldn't upgrade it too much because of lack of replacements.
You sound like you have things sorted, but some important things I discovered about these rifles:
1. Upgrade the hop unit to PDI (although not sure if the A1 PDI hop was good, there was a crap PDI hop replacement for one of the SP's). If it's like all the other ones on the PDI website, it's good - the XB hop unit is made to fit in a 1" hole - it's about 25.2mm diameter. Make a new one piece arm. Replace the o-ring with a rubber band and if you can, fill in the two small adjustment screw holes with JB Weld and tap for one M3 grub screw - this makes finding replacement screws and allen keys much easier.
2. Not so essential but you can replace the trigger and sear. The sear is fairly easy to remake from steel, a hacksaw and some files. I still found the trigger pull to be really heavy, but smoother, with the K2 bearing trigger and sear so I made one that looks like the one on PDI's site which basically acts like a zero trigger.
3. Springs - I use L96/APS2 springs and my stock spring guide is 7mm and upgraded one 9mm.
4. Measure stuff (with a decent vernier). This is a great rifle and most of the parts are machined so are easily replaced or remade. If you move away from "this part is made for this gun" marketing, you can find stuff that fits.
5. Guarder made weaver scope rails to replace the odd-sized stock ones. VSR rails also fit but you need to make the back of the receiver rounded. Real ones fit too but you may have to retap the holes (see the AirsoftRetreat post below). The holes in the receiver strip easily - I have permanently attached my rail with JB Weld.
I am currently making new parts so if you do need a new trigger, hop unit etc. let me know.
PDI's store: http://x-fire.org/etop/m40.html
This is a good post on AirsoftRetreat: http://www.airsoftretreat.com/forums/index.php?topic=97842.0
Some other stuff I wrote about XB's: http://www.airsoftforum.com/board/lofiversion/index.php/t159286.html