If you are using an auto it really depends on the stall converter you are using. However if you are using a manual it doesnt change. Driveline loss is a bit of a misunderstood thing amongst people. Trying to use a % of lose is very hard. Reason is say you make 500 hp with one engine and you work on the 20% rule then rwhp shold be about 400hp. Now you decide to change the motor but keep the same driveline and you now make 800hp atfw, so you use the same 20% loss rule and you now make a supposed 640 rwhp. but its the same driveline so how can it use another 60 hp? It cant, it still takes the same amount of power to drive the gearbox and diff as before.
There you go clear as mud:P