There's not really much you can do, nor need to do to improve these pistollivers. Only thing I might want to do is add an illuminated tritium or at least a white or dayglo dotted front sight element and something like a Hogue Handall rubber grip sleeve. There might even be room for a mechanical trigger job and a way to improve the detent system -
Far as what you did, all looks good.
The only thing I think is not too practical was cutting the bottom off the Co2 holder. If you ever needed to quick change a 12g, you'd be SOL. It's a tough enough job with the small lever for leverage on the bottom in the stock configuration. I couldn't see trying to find a quarter and all that's involved with that -