With the Fullfield II, you can just take the turrets off while hunting, right? Then you can throw them back on for fun shooting.
I like my BPLEX Fullfield as a hunting scope. An inch or two is plenty good enough; I wouldn't mess with turrets while big game hunting.
That might be a good solution. A drop-compensating reticle will do a lot to make long-range shooting at things like water bottles (and deer) quick and fun.
WRT magnification, the .30-06 stock 700 with a sporter barrel is good for what, 3" groups at 300 yards with expensive ammo, off a benchrest? That's all my .30-06 is good for, and that's more than it needs to be.
If you're standing, sitting, etc., there's no sense shooting anything smaller than a pie plate at distance. My 3-9x40mm works well enough that I can see all my bullet holes at the local rifle range, and it's a half-lit 100 yard indoor range. I don't even crank it up to 9x. The reticle just about covers up one .30 hole at 100 yards. Low light is no problem, either.
That Leupold has such fine optics that I truly don't think that doubling the magnification would give you an advantage worth spending a few hundred bucks. You won't see anything new; you'll just see the same thing slightly bigger. In fact, you may well crank the magnification back because the slightest movement of the rifle is magnified, too. This can be fatiguing.
My thoughts: to see a really meaningful improvement, you'd probably have to spend more money than makes sense, and you'd give up hunting utility in the process. And I think you're right to think that you will shoot best if you get to know your one rifle very, very well.
Can you just add turrets to the fine scope you have?
One more caveat... I've heard that many scopes, until you hit the really expensive price ranges, don't return exactly when you turn the turrets, say, from your 300 yard setting back to your original 100 yard setting. I have also heard that Leupold VX-I scopes, while they don't have a "clicker", return quite accurately to where they were set. Unfortunately, I haven't tried it with the Burris I have, since I have the BPLEX reticle that includes tickmarks for every hundred hards, so I can't vouch for how well the adjustments return to previous settings.
(By NO MEANS do I mean this to be condescending in any way, if it sounds like that. I mean this as a set of sincere recommendations. And I figure that saving $400 and still getting the results you want is a good thing.)