I have a '78 honda and the recommended ratio is the same, 25/1 .. but i run at 32/1 because thats what the oil manufacturer recommends. The bike is old and times have changed.
Using more oil than you need has a few drawbacks.
Carbon builds up faster.. (have you already checked for carbon in the exhaust system?).. More smoke.. The spark plug will be more prone to wet fouling.
As far as a rich oil/fuel ratio being related to slower top speed, it may be plug fouling. At least use a new plug, gapped to spec. A new condenser can't hurt.
Another possibility is that lead substitute or any additive might cause some separation of the fuel and oil, compounding the fouling problem. It only takes a tiny drop of oil (or water) to foul a plug, especially if the gap is small.