well if you keep the boost low nothing but youll only gain about 20bhp
to adjust the compression ether use
1. a stroker kit, mainly used to increase the displacement but lower the compression but lower the compression as a side effect
2. fit low compression connecting rods ( often shortened to conrod)
3. of your not already doing so use super unleaded ,highest ron possible, as this will help against knocking due to the higher compression
i see your looking at adding nos to the car why not just use the nitrous rather than supercharging the engine itll save you around £2k
if your going down the road of supercharging and adding nitrous are you looking at keeing the car for a good few years if so look at adding forged pistons while the blocks apart when fitting the shorter conrods
the problem with doing a lot of under bonnet work is that the compression will get too high ( way to much air fuel going into each cylinder then being compressed read the article on turbo / super chargers) ie running with the charger at even 1 bar along with the nos may be too much for the engine to take and end up with you requiring a new engine bit of a pain if your using the car daily
How to add a turbo to a non turbo car.
address for the charging article