Yea, there's plenty of client-side mods that could have a significant impact on a competition server, and you've got almost no way to bypass it.
Heck, REI's Minimap could give a very unfair advantage, particularly if they bypass the restrictions on things like player/mob locations on the minimap.
There's several client-side mods that do things like allow creative-mode flight, speedhacks, teleporting/warping, xray...
I think Vechs had a solution for xray mods/texture packs... it was something like having a layer of TNT in certain areas so all you see is a massive wall of TNT rather than what is inside. He also made a lot of use of Bedrock, which xray mods tend to not squelch as much