If it was not possible to upgrade the brakes on one, I wouldn't own a Dolomite.
Triumph did upgrade Sprints to vented discs......
I take it then that you'd not like the Herald/Spitfire cross we have, which has type 14 front calipers (upgraded by me from type 12s) and no servo. But even the wife said they are good enough. Though it may help that it's hard to get more than 90 from it on the flat - and that with a fortnight's run-up - and I'd just said they could be better.
Was the vented disk conversion from Triumph an ST option? Possibly Jeroen can say what the Group 1B (Group 1-1/2) race cars had fitted.
I ran my GT6 (the one I tried a servo on) on type 14 calipers and messed about with it considerably before I got bored and moved it on. The servo I fitted wasn't the optional tiny Girling Powerstop that was offered by the factory, but a Lockheed remote jobbie from a Hillman Hunter (IIRC) So yes, I probably DID overdo things on that front! It was just too easy to lock it up so I ditched the servo and went back to stock. I actually circuit raced that car back in the 80s and had very little trouble with fade, nothing that fat new discs, some mintex race pads and race fluid couldn't fix. But again, the car was VERY light and fade is a function of weight, grip and brake power in an exotic ballet, the calculation of which isn't really in my wheelhouse. I just know it makes a difference if the car is lighter or heavier for the same stopping power.
The vented discs offered by Triumph/BLST seemed to cover the BSCC championship cars and pretty much anything else if you wanted them. Ken Clarke (I think it's Ken) still offers exact replicas of the ST originals and bits to convert the calipers. The price is eyewatering, but they are available if you need accuracy for your homologated Goodwood Revival racer! Even using these, there are, on Youtube, several period films featuring the Sprint racecars with Gerry Marshall and others being less than complimentary about the Sprint's ability to stop on a regular basis!