Is this a good plan to upgrade wheels then tires?

I have duratracs on a 97 Nissan 4x4 and I love them too, but they are loud. I'm not sure I'll put them on the Tacoma, but I'm glad to know 265 75 will fit.

 

Depends if you 4X, If you do decent to heavy 4x 16" wheels are better than 17", you bet more sidewall to work with. This is the reason the TRD off road come with 16" rims and the sports come with 17"s. I am running the FJ trail teams wheels, 16" with 33" duratracs and really enjoy that set up, especially with the 4X I do.
If you aren't into that then go with you really want

 

I really don't think a 1/2" of extra sidewall per side of the wheel will make any difference in off-road performace.

No one can be 100% sure why Toyota decided on those sizes without talking to the person who made that decision, however I'm willing to bet it was more about looks rather than performance.

 

Do you get worse gas mileage with a bigger tire.

 

Yes, in general, especially if it is a lot heavier.