To be honest you won't see a huge gain as apart from the engine / box itself there is not a huge list of differences between the Sporting and GT.
If I were in your position I would be looking more at costs to insure, costs to run etc...
If you went down to an 8v they are incredibly simple to work on engine wise but you'll find it gutless without investing more than the value of the car on it.
If you can run a GT get a GT tbh, if a sporting, then get a sporting.