any restriction would be the wrong wording ... every thing has a restriction, the consistence of exhaust flow is what changes spool times...
also there's a whole host of other reason why outlaw class drag cars run turbos the way they do. And none of them have to do with a optima.
Unconstipate the exhaust flow to, through, and away from the turbine housing outlet, and the boost onset will improve. i.e., you'll get into boost sooner [onset threshhold], and get to a higher boost level faster, if you lower (not eliminate) the backpressure in the exhaust post turbo.
A larger downpipe/exhaust will net you a SLOWER spool, not quicker (mainly a larger downpipe). It will however boost more on top in since you are freeing up the restrictions. It is kind of a trade off when doing larger exhaust. You trade some low end for more top end HP.
Same concept when doing a custom intake manifold and changing the size of the intake runners.