Let's see some calcs about Rock Polish Genesect, against the top 20 more common pokes in Ubers. I will be using Bug Buzz + Thunder + Ice Beam as the 3 moves, as i believe that Flamethrower's use is very limited (you will see why), and all the other moves are essential for Genesect. All calcs are assuming a +1 SpA boost:
As you can see Genesect can OHKO pretty much everything after SR + Spikes, except from Ferrothorn, Arceus, Dialga and SpD Giratina-0, and 2hkoes the whole top 20. And out of the 4 pokes that Genesect can't ohko, only Arceus is likely to be at full health late game, as Dialga, Ferrothorn and Giratina are all easy to weaken, as they don't have reliable recovery, and are also used to check many other pokes. Also from the list you can see that a fire move has little use, as it is only useful to ohko Ferrothorn and Forretress in Sun, while both Thunder and Ice Beam are essential.
So far, Genesect seems as a top-tier cleaner/sweeper, but we have one more thing to look at: how easy is it for Genesect to set-up and gain the SpA boost? I will leave this one for you guys to discuss...