I understand why your trying to do this chuckles I really do, there has been a lot of nerfing on the tyranids older models, however I think the only point they fall down on is a lack of effective ranged AT, but I believe this is a design flaw with a purpose, tyranids are supposed to swarm their enemies, ripping them to shreds with brutal ease, which is something they do manage to do very, very well, genestealers are the most brutal assault unit going, and not having assault grenades on them is there for a reason, so that they aren't ridiculously good all the time, you can already turn up and assault anything up to 18" from the board edge, I'd call that a pretty darned effective capability without making sure that every now and then the unit they're charging doesn't have some kind of advantage, they rip apart and infantry unit in the game with ease, personally I think they're fine how they are.
On the hive tyrant, let's compare him to the GKGM as they are similar costs right now, a hive tyrant has 2 better S,T, an extra wound, 3 more weapon skill, can break tanks open easily and an extra attack. His tradeoff is a worse armour save, no invulnerable save, a lower ballistic skill and a lack of grenades. They can both inflict instant death, the hive tyrant has superior psychic powers, and he has the capability to be a hell of a lot better at shooting. Now in a 1 on 1, the GKGM will win, I understand that, because he can be geared to win with his invulnerable save helping him massively, but my point is each has its own role, and the hive tyrants certainly is not running up to a FW toting hero that can take a beating before lopping its head off. The hive tyrant is fine imo the way it is.