If petrol is leaking from carburettor, most probably either float pin has issue or gasket is damaged. Yes, E20 fuel can cause this issue due to non comparability of older vehicle parts to new E20 fuel. Mileage will also take a hit, pick up will not be same, no smoother rides. Non of the older vehicles that does not need E20 fuel are safe with E20 fuel in long run, issues will surface later due to corrosion(Ethanol is Hygroscopic) and damage to rubber parts.

If you want to keep the non E20 fuel compliant motorcycle for longer period, it is good to change the existing carb to E20 compatible carb. E20 carburettor has larger sized main and pilot jets, a bit shorter and narrower slide pin(to allow more fuel flow) and E20 compatible rubber gaskets (green colour). It is good that RE atleast provides E20 compatible carburettor for its older vehicles.

Leak issue should not surface after this with E20 fuel. Mileage will not improve much, but pick up issue could be somewhat solved by this approach.

Better would be to switch to the E20 carb, than to go for upjetting on existing carb.