Depending on the engine's size and state of tune you may or may not notice a drop in power, as no stock style exhaust (and that includes the Vintage Speed) is as well engineered as a proper 4-1 header/extractor. A1 does make their Sidewinder tucked up a little higher for Speedsters (it's an option when buying from Aircooled.net) but how much more ground clearance it gives, I don't know.
PS- do you have a pic of your Speedster to post?
Don't raise the rear- drop the front 
