Standard cylinder = 132cc....XL cylinder = 162cc

It's a linear relationship between cylinder volume and shot count so the XL will give approximately 20% more shots (82 > 100 using the manufacturer's figures). The larger cylinder obviously moves the balance forwards a little, no bad thing IMO.

The stock upgrade. Daystate get their stocks from Minelli, they select those with particularly nice grain/figuring and put them aside. It is these that are offered as an upgrade, not cheap IIRC (circa £200?) and functionally no different to the regular stocks. The regular stocks tend to be a grade or two above most manufacturer's so it comes down to how flush you are. If you are buying in a shop find one with several Regals and chose your favourite stock, a good dealer will be happy to do this.

If shot count is important then perhaps look at fitting a regulator? You'd get slightly more benefit from the larger cylinder as the regs result in a small amount of lost volume in the cylinder. XTX Air offers this service, and may be worth considering instead of a stock upgrade? HTH