98 Octane fuel gives me 12L/100km, E85 gives me 16.5L/100km.
It is claimed that E85 is 30% less dense than normal fuel, therefor is 30% less efficient - however mine used 37.5% more in same driving conditions.
However, regardless of what people say, I noticed on my second consecutive tank of E85 a significant power increase - my car was activing traction control up to 80k/h, where as with 98 octane it will only activate up to 40km/h.
Most of my driving is freeway, or roads that are 80km/h speed limits with minimal traffic lights. (minimal stop start on the freeway)
As for whether they run better on 98 compared to 91 - don't know, never tried.
Will the engine develop issues - depends how it's treated and what is done to it, every engine has a life span.
My car feels sooooo different on Caltex Eflex (E70-85) compared to BP 98. I can't really say if I can feel the extra power, but agree with what you're saying re the traction control. Sometimes mine skips sideways taking off from a standing start in 2nd gear (Auto), and that's only say half throttle at most.
The most surprising differences though are the engine and exhaust note (far quieter), and the auto changes (far smoother shifts and less downshifting). On corners I go around where my car would have dropped from 5th to 2nd with BP98, it now stays in 4th or at worst drops back to 3rd. The only conclusion I can draw is there is more bottom end torque so the downshifts aren't necessary. No pinging or laggy acceleration either. It gets up and boogies, and even under light acceleration just pulls away smooth and strong. Note my car was tuned to run on both 98Ron and E85 by Gentech.
The ironic thing is the push into the back of the seat seems harder with BP98. Pperhaps that's just a perception due to the V8 growl my car makes on that fuel. Unless I virtually floor it with the Eflex the engine note is very very quite - almost back to standard note. However I think it's more to do with the tune. My view is that the Eflex torque curve starts to climb lower in the rev range, climbs rapidly then plateau's, whereas the 98Ron torque curve is lower at lower rev's through to the bottom of the mid range, say 3-3.5K revs, then climbs above the E85 curve. hence the feeling of being pushed hard into the seat, and the harder sharper gear changes at high revs. Just my theory though.
By the gauge I used a little over a quarter of a tank to drive Canberra to Wagga (250k's). It was full to the brim of E85 Eflex.