A Viper with a stock motor should run just fine on 93 octane.
As to MPH, I have noticed my MPH vary between 113 on a really bad run up to 120 on my best run, and I am fairly consistently running about 118-119. I believe my launch technique and shifts have the most to do with the MPH. If I launch at too low an RPM and bog off the line, my MPH will be low. If I launch at too high and RPM and spin like crazy off the line, my ET will not be very good, but sometimes my MPH can still be high. An ideal launch, with just a little bit of tire spin and no bog is best, but hard to achieve and depends a lot on track prep and conditions and mostly on the driver. Additionally I have noticed that if I don't shift VERY quickly and get back on the gas and out of the clutch I will lose MPH. I have not had the guts to power shift it through the whole 1/4 mile yet, but plan to try next time out. Also your shift point affects MPH. I am sure every Viper is a little different and most people swear by shifting a stock GEN II at 5,400 to 5,600 RPM, but I have analysed it. Using my dyno graph, and gear ratios, when my engine was completely stock I was better off shifting at 6,000 RPM in 1st and 2nd gears and at 5,800 RPM in 3rd. I have since added headers, high flow cats and cat back, as well as smooth tubes and filters and redyno'd. Now I am better off shifting at 6,000 to 6,100 in all gears as the motor breathes much better at upper RPM than it did stock.
Hope this helps.