I'd say that if it's possible to get it in the game without using console commonds or exploits, it's not cheating, at most, it might be unbalancing. But since you're using mods, it's possible that you could get more defense than you were ever meant to be able to have in the original game, depending on what mods you have, since I don't know, I can't say how high your defense can go.
Though, I'm not sure how much is really a reasonable amount. Rather than naming a specific number, I'd say the best answer as to how much DT is too much is "Whatever makes the game feel too easy." So if you feel like your game is too easy, than I'd say change to weaker armor, unless you want to feel unstoppable, and if you did, I'd think you wouldn't be asking this. Though I'd still keep around whatever you're wearing now anyway, leave it in your Lucky 38 Suite, or your Novac hotel room if you haven't aquired the former yet, or something like that. Because while it might seem too powerful right now, once you start going up against stronger enemies, you might end up wanting that extra defense.
I'd feel like a fool running around the desert in Star Wars stormtrooper armor.
I'd feel like a fool running around in a world of high-tech power armor and plasma rifles wearing nothing more than a suit made out of tanned animal skins, myself.
More than you can get in the vanilla game is probably cheating, so take it from there I guess.
What if your enemies also do more damage than in the unmodded game?