While I personally hate playing against snipers in Turbo Mode, I think that modifying Sniper damage just because of its lethality in turbo mode isn't the play. It would mean that snipers would also have less damage in normal vanilla rooms, discouraging its use. Effects to gameplay in vanilla rooms should have more priority than effects to turbo room or any other fringe/custom room type.
I think the best compromise is to go with what Omma said about being able to ban specific weapons. While tbh this is just a ploy by sprayers who want free kills in turbo rooms but don't want to be killed so quickly by snipers, it's still a nice feature.
All that said, while I don't think snipers should make less damage, equipping snipers imo should affect movement speed. They're long, they're heavy, they do tons of damage, and yet when you manage to sneak up on them they can easily just wall post away and quickly reposition themselves for a counterattack. This means that not only are they useful from a distance, but they're also useful when you're close as well. The main purpose of this is to add some consequence letting your opponents know your location, as well as to and encourage a gameplay that's actually intended for snipers: long range shooting.
At least that's my 2 cents.
