My understanding was that all of the fastest modern sport bikes, like the Hyabusas and R1s, use caged balls just like the GP racers. GP racers had complaints about "wedging" where the inner bearing would tighten in the cone momentarily.
There are two problems with fitting taper rollers to a Vincent. First, you have to use a modified existing bearing or a smaller taper roller bearing. I chose to use a smaller bearing because I was making a new headstock also. The 32005 bearing used on Ducati 900 headstocks was my choice, but it requires a shim or short sleeve in the Vincent steering head to fit. This is not ideal, but David Tompkins used my taper-bearing steering stem for racing for two years with no issues.
Second, with taper roller bearings, the stem itself is used to locate the taper bearing. On the VOC conversion, this is done by using Loctite to position the bearing on the stem. The stock steering stem is not a precision machined item and it is often too rough to be used as a locater for the bearing cone. On my taper roller steering stem the stem is machined as the Ducati stem with two positions on which the bearings locate.
Otherwise, it is pretty straightforward.
I used a 1" x 20 nut for the stem because it was easy to get. The top hat flange was made to fit under the FF1 to take up space. The bearing shims are sitting in front.
David and I had both experienced broken stock bearings in the lower bearing using loose balls. I suspect it was cumulative damage to the bearing pockets and/ or bending of the steering head over the years.
David