Higher voltage does not inherently provide any more power but for a given current, higher voltage provides a linearly higher power assuming a zero power factor (i.e. pure resistive load). Safe wire guage requirements depend completely on current. What this means is if 12awg wire is considered safe for maximum current of 20A for the given length run, then having 240v on that circuit would provide twice the power as compared to a 120v circuit. So, given real world limitation and conditions, it can be said that more power can be delivered using higher voltage. As for efficiency, you have to look at the problem in another way. Given two 2400W appliances, one running on 120v and another running at 240v. The 120v would be running at 20A whereas the 240v would be running at 10A, again assuming zero power factor pure resistive load. That would result in the the 120v appliance having 4x the line loss (power lost to the transmission line) because P = I^2 * R. With a given resistance, double the current means quadruple the power. That said, the wire R is very low given large enough cabling so even quadruple the power is still very low compared to the delivered 2400W. BTW, this is the main reason high voltage transmission lines are used for long distances. US long distance high voltage lines are > 100kv (or even more than 200kv) which allow for a large amount of power with reasonably low current.

Is the new stove receptacle rated for Cu/Al? If so, you don't need any antioxidant. If the stove receptacle is Cu only, then I would do a proper Cu-Al splice in a junction box, then wire the copper wire to the stove receptacle, or find a Cu/Al rated receptacle. Also, is the aluminum wire #6 for 40A? It may be easier and cheaper to just rewire an 8/3 copper run from the panel to the stove receptacle and be done with it. The best antioxidant that I've seen is Penetrox A-13 by Burndy. I don't think the big boxes carry it. You might get it mail order online. I get it from my electrical supply house.

Here's an intriguing option: use the existing 240V wiring for one of the new over-the-range GE Advantium 240 microwave/speedcookers/convection ovens. Reuse the 120V/15A wiring for the gas stove ignition. The Advantium 240 line was just revamped. The new ones look and function as regular microwave ovens, but with 5500w available, can also double as a regular 2nd oven, or triple as a high-speed oven that combines microwave and halogen cooking to create quickly-cooked dishes that are crispy and don't have the usual soggy microwave taste and texture. You would need to downrate the breaker from 40A to 30A, but otherwise it should work easily.

I've been wondering why they didn't do this a long, long time ago. We've got a lot of older grandfathered 3-wire installations, but code now wants 4-wire receptacles, with ground/neutral seperate. Alright, fine, but I always wondered: Why do we have 120/240 appliances anyway? Clothes dryers tend to have a 120V motor, but I can see no reason not to use a 240V motor instead. I can't see anything in a typical range that uses 120, with the possible exception of some of the electronic controls on modern ovens, which are low-voltage internally anyway, so if you're gonna use a 120-to-whatever transformer, why not a 240-to-whatever one? It would seem, to me, a lot simpler a solution!