First, the V8 swap into Miats has been going on almost since they came out. Panache looks to possibly be the same company that used to be called Monster Miata (same location, engines, etc...).
Second, $15k for a SVTF with 25k mi seems awfully high considering many dealerships were selling them new at that price. They had a very hard time selling the cars for the MSRP. $11-$12k would be a more reasonable price IMHO.
