depends on what year trans you get. Its seeming like if you got the 04+ model you would be under 2k rpms at 80mph or right at it. Which is a hell of a lot better than 3750 that i was at with a 99 p11 gearstack in my b13 lsd trans.
If you get the earlier trans you would probably be around 2500-2700 rpms at 80mph on the freeway.
the 04 + trans is basicly like having the same gearing as a p11 trans with an addition of a 6th gear. Almost all the mph's come out very very close to each other in comparison. Which like i said is nice for me cuz in the 1/4 i can still cross in 4th gear up to 150mph at 8500 rpms or so and not have to worry about short gearing in 1st and 2nd.
Again i cant see any downfall to this swap. As much money as people spend on P11 trannies and still breaking them you might as well save the $1700 or so it would take including the price of the kit, trans, hubs, shifter assembly, axles, clutch, and flywheel. Someone worked it out to those numbers and its pretty accurate for the price the 6 speed trannies go for and the cost of other parts.