Strength caps at 9,999 to my knowledge (you will never get their by ordinary means). All other stats cap at 999. In all honesty, max stats does not break the game nearly as much as a well developed devil's arm.
Somehow I doubt it. Before the fight begins, you give Abbyssion the devil's arms, so unless you had doubles of them, you're not going to equip them during the fight. Even still, I have yet to hear of a cheat or code to enable the devil's arms extra damage prior to beating Abbyssion.
Want a second opinion before disputing that moderation? Go to Hellhole: http://www.gamefaqs.com/boards/573081-hellhole