Noe I'm going mad, replying to my own posts - just had a quick re-read. although no doubt the exhaust is an issue, it's also worth a thought that with a wade 140 cam (which has a fair bit of duration and late intake closure) - that sort of cam really needs more static compression to make it 'work' properly. And one of the usual suspects as far as having insufficient compression ratio for a given cam is that the power levels off/falls away sooner than it should, before it has reached what should be peak output rpms. It's not totally out of the ballpark (such a cam would be a drama with low 9.xx compression, but you'll probably find some more top end (and on 98 octane it'd be safe assuming teh timing is ok and there's nothing unforseen like leaking valve guides or something to contaminate the intake and lead to detonation or pre-ignition)- anyhoo, you'd be good with something more like high 10s, even 11.0:1 . Obviously sus ut the exhaust side of things first, as it might likely respond with enough of a power increase that raising teh comp ratio just doesn't matter.

I suppose another little thing - make sure the exhsust flanges on the extractors are aligned to the exhaust ports, and if need be use a die-grinder and open them up so the holes in the flanges are about 0.5-1mm larger than the port exit. It not only prevents any obstruction to the flow (which many off the shelf extractors inevitibly do without a little attention) which will be more and more of an issue as rpms rise, but also the anti-reversion step - it'll actually come onto the cam sweet spot at a lower rpm. Having your cake and eating it too.