Ex chambers are a waste of money, even more so if you put it after the standard reg. If you put it after the reg the co2 will expand even more after the its been regulated, causing variances in the incoming psi to the gun. This will or could cause your feet per second to be all over the place when the gun is put on the crono. 256, 272, 263,280.
My suggestion is to pick up a palmers regulator. The keep out co2 and it is a very consistant regulator, even if you eventually switch to nitro. I'm picking one up my self tonight, F@#$ING pmi regulator. Actually to tell the truth pmi's reg is not that bad at higher pressures, just needs a couple extra repair kits. The palmers is way better though.