Cubes, like anything, they are good up to a point. They are certianly good for response. Either way by getting an aftermarket one you are only gaining say, 5% power at the very best so value for money aint there and you have to deal with the tradeoffs (heat/not stock looking/worse response for equal lengths etc).
Turbo Tits, if boost is falling away its more likely the compressor is running out of puff. The manifolds increase the engine's volumetric efficiency, rather than the turbo's capabilities. There might be some gains to be had on the midrange but who knows... Remember you can have the primaries as large as you like but the exhaust pulse still has to squeze thru the relatively squashed turbine housing. Sorta like a storm drain with a garden spray jet on the end.