There is only one requirement for a Feistel round function and that is a good diffusion and confusion. It is not required for the round function to be invertible in a Feistel network. You can use (as asked) a secure mini SPN or even a hash function (Sha3...)
it doubles the block size, so the number of rounds can be doubled at no perf cost
If you meant to use that $n$-bit secure SPN (such as Rijndael) in your Feistel $2n$-block-cipher, then you will have to make a sufficient number of rounds (at least 4 to get full diffusion : see p. 41). Therefore compared to a parallel CTR encryption method just using that $n$-SPN, you are 2 times slower and you don't really have a gain in security in term of key as the space is the same.