So it appears that removal of non-optimized path results in no downtime, using software takeover also results in no downtime. Removing optimized path/paths, rebooting controllers without takeover or any other unscheduled failure along that route results in a 30 - 40 seconds delay. Much bigger loads on the system I expect the delay to be nearer 60 seconds.
So in conclusion to do any maintenance we need to ensure that we move the aggr's from the affected node using takeover before and then disable automatic takeback to ensure that it doesn't automatically return to their home nodes. These timeouts appear to be down to the ALUA protocol and in my view in effcient!