Willis I don't want to dampen your enthusiasm or anything like that, but with all the thousands of lawn mower racers out there in the world, and all the folks that have asked all these questions concerning hydrostatic trannies here on this forum, let alone all over the Lawn Mower Racing community as a whole, don't you kinda suspect all of what you're suggesting doing has been tried before?
As for your reverse theory, I can go out and put my car in reverse and get it to go backward about as fast as it takes 3 gears to go forward, without too much effort! And, the gearshift doesn't move any farther there. Reverse is geared differently whether it's a matter of teeth to teeth or the direction and rate of flow of the fluid through a hydrostat.
From a theory of simple hydraulics, any time you increase the rate of flow (a.k.a. velocity), through an orifice such as a pipe, or the vanes in your tranmission, you also have an inherit increase in friction. This is caused by the fluid scouring the walls on the pipe or the vanes in the tranny. Where the pipe is designed to handle a certain velocity, there is a nice little layer along the walls that moves along at a slower rate than the fluid in the middle of the pipe, acting somewhat as a cushion for the fluid moving through the middle of the pipe, thus reducing friction. When you increase velocity, forcing a greater volume of fluid through said orifice, that cushion is lost.
Friction as we all know generates heat, and therefore if you increase velocity, thus increasing friction, you need additional cooling capacity to keep your tranny together, which you don't have on a mower. When the tranny fluid gets too hot, which it will if you increase velocity without increasing cooling capacity, it loses vicosity and doesn't lubricate the parts in the tranny properly, at which point things start to fry!! I'm not going to dispute the old school tractor pulling dude's theory on removal of the restricting orifice, he's very much correct it will go faster if you take that little gizzy out of there, but... tractor pullers don't run 10 - 20 laps dude.
I can also tell you that having worked on the hydraulic system on a piece of equipment or two in my day, that when you put it in gear, you might want to have it idled down a bit, because that little restricting orifice you're talking about removing also effects the rate at which it will take off! Meaning that if you have it wound up tight when you slip it in gear, it's subject to have a tendency to launch on you guy!
It's your tranny, and if you want to toast it, that's alright by me, but given resale value of a good hydrostatic tranny, you'd be better off e-baying the hydrostatic and using the cash for transaxle or a Peerless 700 and Kart axle set up if you're serious about racing it. If you're just looking to bounce around the yard with it, take the blade off, put a helmet on, make sure there are no small children or pets in the way, get your insurance policy paid up, and have fun while it lasts.
