While it seems to be advantageous for players to use speed-focused mods on their characters, the extra speed undermines the effectiveness of most other mod stat boosts. A squad of characters with low gear but high speed can win in arena every time against high gear squads with lower speed. In fact, it doesn't even matter if your squad can do significantly more damage, and should be able to out-class them, the faster squad will usually win because they can attack more often.
My suggestion is to put a cap on the Speed stat at a maximum of 200. This will make the game more competitive, and allow focus on other stats that SHOULD be more important than speed.