Why not use Scrypt as intended. Scrypt with default variables has beyond horrible performance on GPUs. Litecoin developers modified it to make it roughly 128x less memory resistant (using only 128KB total).
Generally rolling your own crypto ends badly.
I'm not a scrypt expert. It's new to me that with the default values it's truly GPU resistant. As far as I understand, it's not that the operations aren't hard to be done by GPUs itself, it's just that enough memory is necessary for a certain scrypt hashing (depending on the used values of course). So with enough RAM a GPU should outperform CPUs like they do it on SHA256, no matter what variable values are used. Correct me if I'm wrong.