10*C shouldn't make that 100 fps difference between a 12ie and hpa. I suspect it's your pierce pin - either it's not puncturing the cartridges well enough or it's plugged somehow.
Are you seeing a reasonable sized hole in the tops of the spent co2 cartridges (ie. the hole is bigger than the hole you see in the tip of the pierce pin)?
Also, are you using a trracer bba? They have a copper/brass (?) mesh in the transfer port (which the gas passes through to get into the valve chamber) that can get clogged over time and may be choking your valve.
If all that is fine, you could start with either a lighter valve spring or the stock hammer to see if that makes a difference. If it doesn't then your problem is likely with the valve getting enough gas pressure.