You are going to get a mix of opinions. Some will 24x, some 36x, some will say all you need is 9x. I like a 16x, but I do have a 24x on my bench rifle and I like to play around with 12x, 9x, 6x, 4x, etc., so I'm kind of mixed-up.
It's an interesting question you have as to what would be the optimum power for a variable scope's optics. I guess it really depends on the design and the glass quality. Some variable scopes, especially lower lines will have issues at their max power due to impurities in the glass and some compact scopes that try to put too much power into to small of a package will have issues on the high end as well.
Great question, maybe someone has a better answer. :t
When shooting benchrest .22 games, you need the power. There are some scope that go higher. Leupold make a 45X Competition Scope and there are several people that boost the power of your scope by 25%. On an ARA, IR-50/50 or RBA target, the bullseye you are aimimg at will fill just about 30% of your field of view.. Of course, it goes without saying, that you don't shoot these rifles from the shoulder, but use a very solid benchrest setup. As far as cost goes, you can find a Weaver 36X for around $350.00, give or take a little..
I keep a spare 36X lying around to test fire all my .22's and 17 HMR for accuracy..