i think since most of the time you're spamming uturn, specs is, ironically, a waste of genesect's power. it's true that part of the reason uturn works so well on genesect is because almost nothing is willing to stay in on its wide special coverage. it's also true that with specs, genesect's raw killing ability goes up. but its uturn doesn't really gain much from the change and you lose a lot of speed. moreover, genesect is a really prediction-heavy mon if you focus on its coverage moves. unlike guys like specs latios that just click draco meteor and go, a poorly chosen special attack will force genesect out for little to no gain, which is precisely why uturn works so well. the lack of stab, and the poor neutral coverage of each individual move, also hurts.
i think if you really want to make use of genesect's broad special attacking pool, expert belt really is the way to go (i would argue it's better than LO, actually, because genesect's classic set is choiced). being able to switch moves is so much more useful than getting some extra power on unstabbed coverage, seeing as being able to select a super effective move will probably be equal to or better than a neutral/resisted specs-backed hit. compare that to a specs-packing dragon (again, latios) where chances are super effective coverage is meaningless and you could just meteor everything, in which case it's not so much about move selection as it is about raw power. i think this is also part of why hydreigon does better with LO/ebelt than with choice items. hydreigon's coverage is excellent (meteor/fire blast/earth power/dark pulse/focus blast/uturn/earthquake/superpower) and it doesn't even need a hidden power, so the ability to switch moves can often make up for the lack of choice boost - the reason being that you have a ton of super effective coverage to compensate with. latios on the other hand has very poor coverage options (meteor/psychic/psyshock/surf/thunder) so it's tougher to score super effective hits and thus you tend to get more out of just spamming your strongest move. also, hydreigon can use uturn more easily on a non-choiced set to get out of matchups that it can't win, which is something it shares with genesect (uturn is also nice for ditching -2 spatk drops from draco meteor and -1atk/def drops from superpower)
idk if my point came across very well in this post...? i guess what it boils down to is that, if most of your special attacking prowess is due to the excellent variety of coverage you carry, you generally benefit more from an ebelt/LO than from specs. genesect has no stab on any of its coverage moves but it has a lot of them, so it's a great example of that. on the other hand, if you have strong neutral power, specs can be very rewarding for that kind of "spam this until you get forced out" mindset. keldeo under rain (is your target immune to hydro pump? Y: use secret sword or switch out. N: use hydro pump), darmanitan under sun, or latios, are good examples of that. hydreigon is somewhere in between because it has a powerful draco meteor that rewards choiced sets but can also smash a lot of stuff with its coverage. i think people generally lean towards LO/ebelt on hydreigon mainly because it's too slow to sweep and it's easier to wallbreak if you can switch moves, but it's the kind of mon that can do any item well. i'd imagine that if hydreigon had just like 5 more speed, choiced sets would become a lot more popular (as well as hydreigon in general >_>)