I think 275-300 rwhp is about the limit you're gonna get with the magnum exhaust manifolds. I do know of a guy who did 430 rwhp, but that was with a LOT of spray.

as close of an apples to apples comparison between the 2, a Dakota with tuning and a performance intake, stock exhaust manifolds and a cat back system made 237 rwhp. same combo with mid-length spin-tech headers made 253 hp.

But, beyond that point, when one starts doing heads, cams, strokers, etc. the spin-techs really start to shine, or rather, highlight the restriction of the stock manifolds.

Of course, all this was on modern EFI Dakotas with Magnum heads too. The Magnum outflows most of the regular production LA heads out there.