Re: Front and rear same better?
if you ahve a non staggered set up, then your total tire width will be less (and also lighter), hence less rollnig resistance. so yes you will save some FC, but hwo much i couldnt say.
similarly, if you are not talking about power that will easily induce wheelspin with skinnier tires, i think you will have equal if not better acceleration with thinner tires, simply due to weight savings.
as you have a staggered set up, theres no real need to change all at the same time as you cant rotate the tires anyway. as long as there is decent tire thread, i would use them a bit more.
of course if your rears are almost worn, then yes you will have reduced grip and this MAY result in possible oversteer. but again, it very much depends how you drive, and what kind of tires you ahve.
out of interest, what model car do you have?