They still have access to 1000 BHP for short periods, while weighing less and having less downforce that will almost certainly result in less drag for most cars. I would not expect peak speed to be that much slower than now, just average speed over the lap.

Sure the drop to 6 gears from 8 (if it goes ahead) is going to hurt but not being able to use all 1000 BHP on the straight at somewhere like Monza means you'll just be a sitting duck all day long, RB have shown that when they had their biggest DRS advantage.

Honestly I expect the overall laptime to be about the same as current after a couple seasons. I'm just guessing too though.

They'll probably gain some speed in low/medium speed corners, and lose speed in high speed sections. Although additional active-aero will help some.