Hi peter from experiance after I had truly destroyed 2 impellors with stones I think they should be a very very close fit, but not touching. Thats why they cost me so much to replace. On the modern jetskis there is a 'wear ring' built in . Basically it means that when the impellor spins less than perfectly you wear away a £50 part & not a £300 impellor or even more expensive tunnel.
Possibly the bearings are stuffed on the shaft meaning its not turning centrally ?
Any intolerance in the gap means the pump runs very inefficiently - this was seen on a mates Jet drive boat that was a 'cheap deal' until he had to get the whole lot machined up to tolerance.
I'm sure there will be more knowledge on here soon.