Cross overs are like headers on a performance car. They can increase flow during the In/Ex overlap and help pull more air fuel into the chamber, displacing more exhaust, thus creating more power. However, how effective they are is all about the length from valve to junction and cam overlap and it would seem unlikely that the Vettermans are tuned (pipe lengths changed to reach max torque) on a dyno, like performance headers are. The space limitations likely dictate where the junctions are.
I believe that Larry Vetterman tested these years ago when he developed them and reported a performance gain, but don't remember the numbers. probably still on his website, as it used to be there.
Straight pipes can be more efficient at a lower power level. Pulling more air into the cylinder and displacing more exhaust during overlap does increase power, but also allows or can allow more of the air fuel to leak out the exhaust port due to the extra flow. In the performance world, the only way to get a full air fuel volume in the cyl is to bleed a decent amount of waste into the exhaust. When dealing with efficiency or emissions, designers find other ways to increase power that are not as inefficient.
we're likely dealing with small differences here.
Larry