Can rotating the tires extend their life by 20%?

So I'm looking at BFG G-Force Sports for my next set of tires.

Ordering online + installation charges would be a hair over $400
total, if I did all four tires at the same time.

Or, the local Discount Tire can do it for $467 including free
rotations for the life of the tires.

You see where I'm going with this. Is it worth the extra $65 for the
free rotations. I figure I would want to get an extra 15% wear at a
minimum. Or, the dealership can do it at services for an extra $25 -
but if I'm rotating them more than a few times, might as well go with
Discount Tire.

Maybe I should add, there's a good chance I would autocross these a
few times, as a novice. I guess that might lead to more uneven wear
than normal.

As an aside, how much effort is involved in learning to change the
wheels yourself on a Miata - or a Jeep for that matter? What tools
would I need to get?

TIA & ZZ