Have you rebuilt your L28E with forged pistons and H-beam rods? Lowered compression?
In my opinion that's too much turbo for the unmodified S30 chassis and "supports 500 HP" sounds good - but is it? In an unstiffened early unibody Z - probably not. We saw a 300HP RB25 swapped Z here in Phoenix tear the spot welds apart. It was a 240Z chassis and the 280 is a bit beefier but still. This is a town with no rust.
The kit looks legit however (I'd still go for a different turbo, but that's me).
I'm not a fan of the charge pipe config that an opposite-side intercooler requires but it's a little more difficult to get same-side charge piping through the radiator core support. Don't get me wrong, I'm in the TURBO EVERYTHING camp - but do your research, and do it the right way, and you'll have a Z that's FUN to drive and will stay together while you're doing it.