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It's OK for me at 15st, just don't use superlight spokes if you're worried about stiffness. Sapim CX or DT New Aero or similar shaped Pillar or CN will fit the On-One 24h hubs, or use DT Comps or equivalent if you don't want to splash out on bladed. I never had any question about the Ellipses with only 20 spokes, and that was when I was heavier and riding the tiny Calshot track which generates bigger loadings.
Very little, if any.
Are there any metal tub rims out there at all? Not really. In the olden days, we had dozens of choices readily available (Mavic, Wolber, Super Champion, Nisi, Ambrosio, Araya, Campagnolo all had numerous different models), now you can count them on the fingers of one hand.
To be honest, for general track use cheap beats out all other factors. Any rim will be strong enough in 32h, none of the metal ones will be much different from an aerodynamic point of view, and they all weigh the same for practical purposes, from Ambrosio Crono F20 at ~360g to Kinlin at ~430g is not going to make any difference.
If you actually want faster wheels, the cheapest change to your current spec would be to get 24h rims and hubs.