The 2" Bayco is $219 from us, maybe you Can do better from your local supplier.
The valves are more precise, but they still have a little "creep-age" as the spring gets ready to pop.
btw: a 15"hg valve has a small give or take, probably 1/2"hg, and then there is plumbing to consider, so yes there is variability.
A 15" valve is designed to NOT EXCEED 15", it's safety valve, the average 15" valve off the shelf will likely start to open about 13.5" which is a moot point, because the action of one of these valves is safer and generates more net vacuum at the wand than a conventional spring relief.
Now, that said, the smaller units seem to benefit more form these valves, big units are over-engineered anyways, so you have air to spare, and it's not as noticeable.