Looking through the old Trapeze magazines, my favorite parts were where FD owners would show their upgrades... mine isn't a speed increaser, but safety.
I'll be the first to admit it... I hate trailering at night. The lights are weak, they flash like they're anemic, and I end up putting reflectors all over the place to make the boat more visible. Goofing around on ebay looking for trailer lights, I stumbled across a number of products that allow the lights to run directly from the battery in parallel instead of in series. I bought one for a screaming deal ($12) and installed it on my Volvo.
The difference has been nothing short of amazing. The lights are as bright on my car, and when the brake lights come up they can actually be seen. Blinkers FLASH, not go from dim to a little brighter. It's done amazing things for my confidence other people will see me at night, more importantly see my boat behind me. Again, just my 2 cents here.