Tender springs just on the rear?
 

Since I'm going to change rear bumpstops I'm contemplating swapping the 325# springs for a shorter set of 335# springs and a set of 150# tender springs. Is there any potential issues with using a set of tender springs just on the rear of the car?

Everyone I've heard of that put tenders/helpers in the rear only made the car push like a pig and fixed the issue by putting them in the front as well.

That makes sense due to the spring rate decreasing when the tender is active.

Whatever you do, do follow the 949 recipe. They don't know shit about making Miatas go fast.