Since the first rifles with the 5R rifling began coming out people have been saying they are faster than the barrels they previously had. There seems to be at least some truth to it and I read several articles explaining why that made a lot of sense to me.
The short version is that yes fewer lands and grooves but more particularly the rounded edges vs sharp edges gave more velocity with less pressure due to lowered resistance, at least that was my take on it.
In general I think that starting off with a quality barrel with fewer imperfections in the lands and grooves makes more of a difference.
The more tooling marks you have in the barrel the more friction between the bullet and barrel so the cleaner it is cut and finished naturally the faster it's going to be.
Interestingly throughout firearms history different manufacturers have tried everything from 8 grooves to 2 grooves in the quest for accuracy and velocity. Today you can buy a four groove from Shilen, the 5R from several manufacturers under license to Obermyer, Polygonal from several, and six groove from nearly all of them.
In the end I think what matters most when it comes to velocity is how cleanly the barrel is rifled no matter the process and most importantly how well it is finished with the lapping process.
Another thing to consider is that the Highest MV can actually be achieved with a "smooth bore". The best modern example is the 120mm cannon on the Abrams tank which can fire projectiles at around 5,700fps.
