Making it less effecient even if your going to use more power than needed isn't necessary or ideal, IMO.. A smaller box needs more power yes, but a smaller box wont give you more overall output.. Build the box based on the frequency response you want, power is just power.. Making a smaller box and using it because it's less effecient is literally saying ok, I want the sub to be as quiet as possible any given amount of power, that way I have to use every watt at my disposal even to the point of overheating the coil just to get it loud... That way I can say I put 1300 watts on it! 3.5 is likely to have a bettter response than 2.75, which no matter how much power you put on it is probably gonna be lacking on low end.. I'd do 3.5, keep a subsonic filter just below tuning and if you hear it having mechanical issues, back off. Putting it in a smaller box will stop the mechanical noise sure, but only because the sub is quieter since it's now less effecient. Having too much power is a good thing, it's better to run into mechanical limits than thermal, especially when it comes to a daily driver where you want longterm high SPL.