I don't think there's going to be a simple way to change the priority on a permanent basis, but in normal operation there shouldn't be a need to.
"Speed" is not really dependent upon the priority - only how easily it will give up the cpu to something else that wants it. If you increase the priority, it may cause other things to have to fight to get the cpu time they need and it may adversly effect other cpu intensive processes that you run.
All that said, you can easily change the nice level of the seti process from within top, however each new work unit starts a new instance of the seti executable - so each new work unit will revert back to nice 19. There is no way that I know of to get boinc to start the process with any nice level other than 19.