I would say yes there is an advantage, it's less restrictive to air flow, imagine blowing the same amound of air through two different bore tubes, the larger tube gives less resistance. The best cover to use other than the RS500 is the EECIV (used on the small turbo escort cosworths), this has been dyno proven by Mike Rainbird on SCS's dyno to be the best of the rest. But as you already have the RS500 stuff then you may as well utilise it! It would be silly to use the throttle body on a 4x4 cover, you then have an RS500 plenum cover that is useless, as it is the throttle bodies which are rare as you know what

As for the injectors, I would say no you won't get similar power at lower boost levels, the boost levels will need to be the same as for the 8 injector setups! the injectors just flow more fuel so you can use less of them, the more fuel + air the more power you get You may (depending on injector type) need to change the ECU to do this, as the Webber Marelli based ECU's can't cope with most of the newer injectors (different impendance types), there are a few that do work though

Do your regulations specify turbo size also?