I'm using those stub axle stiffeners. I had to adjust wheel bearings after every super sprint, just used them in the last round and didn't have to adjust them so I feel they are a positive. Using the machine to size type. As in feeling on the track couldn't tell the difference but the theory behind them seems sound. My car gets around Morgan Pk Raceway Track E at 1.07.15 which is not hanging around as well as smashing plenty of curbs. Do it well help wheel bearings survive. Darrin.
Trying to understand I came to the following conclusion. the basic ones use standard bearings and fit between the inner races stiffening the whole assembly. I assume they are a press fit. The same company does a sleeve which fits over the stub axle using the standard hub nut to secure to the axle. Essentially stiffening and increasing the diameter of the stub. The sleeve accommodates a larger bearing set and suits their new hubs. I assume the new sleeve has a thread and nut to hold the new bearings and hub. This also has a bearing spacer incorporated... If I understand it right
I can't see them being a press fit as you wouldn't be able to get the hub off.
As Danny stated you would need to clearance the sleeve as it would impact the pre-load on the outer bearing (and inner for that matter) as potentially it'll hold the bearing cones off the cups if too long, and if too short won't effectively provide the stiffening affect.
They fit between bearing. I had my hub on and kept removing the outer bearing, removing the stiffener and machining hub stiffener in the lathe until correct bearing tension is achived.