Vipers were downgraded to 378bhp (basically their power rating on 8x US octane) because of emission standards in Europe and the UK. However, that number could be brought back up using the highest octane number possible over there.
450 hp is the Viper's rating on 9x US octane gas. The top speed is variable on tire options and weather conditions. No I wouldn't say it's either 177 mph (headwind in hot weather on regular tires) or 193 mph (tailwind in cold weather on performance tires), but more like 185 mph (two-way average).
Many sources state 185 mph as well. I figure that's a more 'reasonable' top speed out of those two figures (one pessimistic, the other optimistic).