Hi
Whilst I love my Bluefin in the wild stuff, I don't think the Bluefin will be faster than the V8 (other than perhaps in BIG downwind conditions where the Bluefin really shines). If most of your paddling is in flat water/small protected waters, I would say look elsewhere. If on the other hand you are looking for a boat that surfs really well and is easy to remount (an often overlooked virtue of the BF) the BF become a serious contender.
The V8 Pro is a nice boat, nice narrow catch, comfortable and more stable than the SF. It does have a relatively flat bottom, which has different surfing characteristics which I didn't like. But that is purely personal preference as I am more used to the characteristics of the more rounded Fenn's.
The SF is a great all round boat. Less stable than the V8 Pro. Once again if you are paddling in safe protected waters with small to medium conditions, you will probably get the hang of the SF in a relatively short period. If however, you are in more wild open ocean conditions, perhaps a step too far and may compromise your safety (harder to remount and less stable than the V8 Pro).
In summary, I think the decision on how big a leap you can take (safely), depends on the conditions you paddle in. If you are paddling in a safe environment, take the bigger step up to the SF. If however, you are paddling in open ocean exposed environments, rather opt for less of a jump and go with the V8 Pro. You can always upgrade again once you feel the V8 Pro is the limiting factor
Hope that helps (BTW, I have owned all the boats I have commented on, so my reference is first hand experience)