Were you using green gas or duster on your marui g17.
With the plastic slide you should be using duster gas, gives less fps( I think about 50-60 less).
I have the PGC slide ( and outer barrel)it works great, I also had a Gaurder slide but the fit was loose( slide into outer barrel had a lot more play than the PGC and I feel less accurate.
I would suggest if you upgrade not to install a stronger hammer spring as it hits the blow back unit really hard and caused one of mine to shatter. You will need a heavier recoil spring though for the heavier slide.