My server, and even most of Xagulz servers we take out speed differences per levels. It is simply unfair when someone has a faster base-speed than you in pvp.
Knights in PvP on RL tibia (good ones) always carry around Time Rings, and if the mage also had a time ring, they'd just give up. Plus, they WERE useless for a long time in PvP, until they were given the spell "Charge" (They were given charge specifically because of the complaints of knights that they were useless in pvp because everyone could just run away from them and they would never hit anyone). Mages could just strong haste away from a knight and the knight would re-log onto his mage 2ndary character because it was impossible to PvP on a knight unless you were Way higher level, or the mage was not premium (couldn't use strong haste)
But lets say on Tibia, I could either increase my speed, my damage, or my armor. Well, i'd probably increase my damage while hunting, and speed in PvP.
Dont' get me wrong, damage and armor do help in PvP, but it isn't NEARLY as important as speed. Speed is everything in tibia, if I could reduce my max hp and mana as a mage by 50% and gain 10% more movement speed I would. Speed matters most and it is overpowered if you can get much faster than other players.
**EDIT**
By the way, you will notice in every single PvP-style game, they limit how much speed you can get VERY CAREFULLY.
League of Legends for example, you can increase your damage so that you can one hit everything, and heal to full hp with every shot, but you are VERY limited in how fast you can get. And most speed boost items are activated, so they have limits. Because all game developers know, Speed in combat, is everything.
Anyways! Good luck on your stat system, You can balance it once you find issues, I was just backing up the other guy that was concerned about Speed ruining pvp.