You aren't going to get a much quicker suit than the Maverick X and on the day of the race, there won't be many people who could afford the luxury of a thermal suit, so they will be in normal ones. 55 degrees isn't too low and 78 is positively tropical!
I have a Helix wetsuit and also the thermal version. I would be using the thermal now as it's only 50 degrees in the sea where I live. If your event is at 78 degrees, wetsuits will be banned in any case! The thermal suit is defo slower than the normal one. Good luck in your event