I ran 6mm and 6.5mm (MW) elements on my SD with stock engine.
The fuel delivery potential can't be used!
Next life I'd start with air supply mods first. Its cheaper, does not smoke and you need it anyway when the IP comes.
On the other hand you spend much money and can not feel it that much.
I was at 186 Km/h (GPS) with my SD, 6mm and stock engine but the big step was with VTG and larger elements.
The vane control is not working well, not finished yet but the first test drives made me

Tom