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After reading through this thread a second time, I'm doubtful the loss in hill-climbing ability had much to do with less momentum in the wheels. How much extra stored energy is there in 2.8 kg heavier wheels? It must be small, and would quickly be consumed by a sustained hill climb.
I suspect something else is going on here. Keeping the same tire on a wider wheel affected something in a negative way, be it toe, camber, or perhaps aerodynamics. In theory, using the same tire on a wider wheel would reduce the overall circumference. This would effectively make for a lower gear ratio and should improve hill climbing ability.
The first thing I would do is have the alignment checked, as I believe that would be the most likely thing that would explain the noticeable loss in coasting and hill climbing. Next I would measure the same tire mounted on both size wheels. Who knows, maybe something strange is going on, and it's changing the effective gearing of the vehicle. Is it possible the lower "gearing" of the tires is allowing the car to maintain a higher actual gear on the hill, which makes it feel less powerful while going up said hill?
All other variables held constant, lighter weight wheels would climb a sustained hill quicker than heavier wheels. The problem is that you haven't held all other variables constant (changing wheel width and tire profile). One of those other variables is causing your problem.
...and it's no surprise that a wider tire handles poorly in the rain. I find hydroplaning to be fun and exhilarating, but I can understand why sane people would prefer to avoid it.
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