RGS#1's estimated top speed will be around 40mph - probably stay in the 20's. As for breaks, I’m using the original transmission band brake, emergency brake, and a separate four wheel friction brake (always good to have redundant braking). T's were/are notorious for poor braking. I’ve installed the larger rear brake drums. I went to great lengths to splice model T rear hubs into 1905 Fairbanks Morse RR wheels and still retain the brake drums. I am also using a full floating sealed bearing safety hub. T's also had a tendency to break axle shafts - thus the rear wheel could come off. With the safety hubs, this will NEVER happen. I'm also working on a pressurized sand tank using a period exhaust cut out (keeping it as period as I can).
FYI, my 1927 TT truck with a Weaver wrecker and worm rear end has a top speed of 20! It has an over/direct/under auxiliary Universal transmission – with this it’s capable of 30mph, or I can walk faster that it when using the underdrive. The 1926 coupe I currently have is capable of 60mph – it has the two speed Ruckstell.