There is a lot more too it than that.
I have usually said it is an
indicator, I have said it a bunch of times and perhaps I don't use the same wording all the time.
Now some of the problems with turning concerning power and drag is that once you are turning you are creating more drag than flying level. As Biff has said this doesn't matter for around the first 90 degrees of the turn (perhaps the plane is trading some momentum as the speed bleeds off).
Some aircraft like the Spitfire and Hurricane with less wing loading can turn using less angle of attack on the wing which means a smaller increase in total drag at a
given speed. The planes with high wing loading need a higher angle of attack and are generating a higher amount of drag in proportion to their level speed drag.
Few numbers from a calculated chart on the Spitfire I at 12,000ft, I chose 23 seconds because that seems to be where the line on the chart is, makes things a bit easier

All turns taking 23 seconds.
2 Gs......................706ft radius..........................135mph..................extra power allows climb.
3 Gs.....................1150ft radius........................220mph..................plane cannot climb but will hold attitude.
3.5 Gs..................1350ft radius.......................260mph..................plane is loosing altitude at around 1600fpm.
Obviously there are occasions where the plane is bleeding off speed to maintain altitude rather than bleeding off altitude to maintain speed.
Having more power gives the pilot more options. It may not change the turn radius much, if any, tight turns get the planes down close to stall fairly soon. Spitfire 1 (6000lbs) was calculated to stall at 225mph at 5 Gs pulling a 670ft radius. It needed to descend at about 18 degrees to maintain speed. Having an extra few hundred horsepower can help maintain altitude or allow climbing while turning or even allow pulling a
slightly tighter radius while not descending.
The 109G-6 gun boat was about 24% heavier than the 109E so a fair amount of the extra power was just lugging the weight around and whatever the F had gained in streamlining the Gun boat was loosing in lumps, bumps and gun pods. And once the heavier plane has to bank and use a higher angle of attack the drag goes up faster than the light plane.
Again, excess power is an
indicator, use it after looking wing loading. If two planes have similar wing loading the plane with the higher excess power should perform better in turning maneuvers. I like climb performance as it is sort of cheat. It takes out the power needed to simply fly the airplane at 140-160mph (or whatever climb speed is) and leaves with the power available for maneuvering. (if the 109 has it's slats out it is down in the climbing speed area)