i wouldn't consider them as replacements for snorlax. lugia is practically unkillable and would make things like suicune obsolete. ho-oh can be a dangerous mixed sweeper.
this isn't RSE or DPP, unless you have a strong physical STAB move (or Cross Chop) you're not really going to make a claim as a dangerous mixed sweeper. Ho-oh of course has awesome stats, but it has so many conflicting problems:
1. awesome HP and Special Defense, but weak to the two most popular special attacks along with a quad weakness to Rock attacks (GSC has never had an uber metagame, but you'd have to figure that Tyranitar would be a big player in it)
2. it has a great attack that it could boost with Curse, but you only have two moveslots left, not to mention Charizard can both set up quicker than Ho-oh AND is faster than Ho-oh.
3. Sacred Fire, like Fire Blast, has 5 PP. burns are pretty meaningless in GSC since everything has Rest and Heal Bell is common. plus, Sacred Fire has 100 base power--I'd use Flamethrower before Sacred Fire. Ho-oh's Special Attack (318) is also nothing extraordinary--Charizard's is 316, for example, Raikou's is 328. it does learn Thunder(bolt) which is helpful, but if we're using ubers, unSTAB Thunder(bolt) isn't doing shit to Lugia (or Tyranitar).
4. this is still GSC. stats favor Defense. there is no Calm Mind. nothing is going to get OHKO'd. or even 2HKO'd.
if you want to sweep with Ho-oh, honestly your best option would be the simple Curse/Rest/Return/Whirlwind--you could use Recover if you have Heal Bell I guess. but if we were in ubers, why not just use Lugia who has fewer weaknesses?
Ho-oh is basically Moltres with uber Special Defense. both have good Attack scores, but no attacks from which to benefit. both have the same Speed and Defense, mediocre among the legendaries/ubers. Moltres has higher Special Attack, even. I wouldn't bat an eye if Ho-oh was allowed in the OU environment.