Alloy cam covers will obviously do nothing for performance. The idea it cures leaks is a bit flawed - cam covers leak due to blocked breathers, poor quality parts, or poor workmanship, otherwise should be good for 7 or 8 years (so 1 set of replacement gaskets in its lifetime).
But if people want that alloy covers, thats fine and the owners choice - but must be made on the facts, not this myth put about by people who cannot fit them that the plastic ones warp (they don't).
If you do fit alloy ones, be aware the next weakest point when the breather system blocks is the crank seal. Thats a bit of a pain to replace, as Vectra owners will confirm.
The multiram is there to smooth out the torque curve. Removal won't do massive amounts for performance, but will make it a more stepped 'surge' with it removed. The genuine GM filter is probably hard to beat for flow.
There must be some gains with remapping, also would be interested in gains from forced induction