How much loss for rear mount turbo?

Critical elements here are turbo sizing, and heat itself.
Rear mount turbo setups usually require a slightly smaller exhaust side, because the exhaust gases lose heat and thereby volume as the exhaust cools down the exhaust system.

Said another way, the smaller hot side of the turbo will require less exhaust flow to spool it. Since the exhaust cools and contracts, a smaller housing will react quicker, where the larger housing that you would use in a front mount system would have lag, and slow response time.