I'm running an RB20DETT setup at the moment using twin T28 turbos.
Lag, well, very considerable. At 9psi, i was hitting full boost at around 4500-5000rpm, now im running 13psi boost and making peak power at 7000rpm...
i GUARANTEE you you will NOT make 280rwkw without some serious headwork and bottom end work, not on an RB20 anyway.
As for manifolds, im using RB26 ones, just using a custom fabricated adaptor/spacer plate blah blah blah, allot of custom stuff