I haven't noticed a lack of engine braking on my 09 XB12R though I've only ridden it and my 03 XB9SL.
Have you had your axel out? After you slip the belt onto the pulley and the fat part of the axel engages the wheel bearing as you torque it down and this tightens the belt.
There is plenty of air gap for the engine to get more than enough air.
This scoop is the best in my opinion and most resembles the left scoop. Though it is pricey.
https://rover.ebay.com/rover/0/0/0?m...2F263520172860