It is true that thumpers present problems when running turbos, and they have a greater tendency for lag than twins do. But that can be resolved the same way as with multi-cylinder engines: use a small enough exhaust scroll to get the shaft speed up early. You sacrifice some in terms of the ultimate boost the unit will produce, but you should be able to get it over ten pounds pretty easily. The problem I see with concealing it under the seat/air box, even if you make the air box able to survive it (might consider replicating the outer covers in aluminum) is that the heat from the exhaust side will heat the intake, and there's no place for an intercooler in this setup. It may cause no problem, but there's something to look out for.