The main difference is the header in the greytop has an extra cat so thats why this one has a redtop header. Some people say that the engine tune is slightly different but no one can confirm it.
I like the idea of more power but I think a light car with rear weight bias is better with a NA power curve. I want the car to be faster but keep its character and mid corner adjustability and I think the on/off boost of the 3SGTE would upset the balance.