Cath and armory reset together and count as one instance. All 4 SM instance count as one actually.

I solve this problem by looting. On my pally I can clear Cath in 2 safe pulls and loot everything in 13-15 minutes. I can do it in 1 pull but will miss some mobs and not get the boss XP or loot. The 2 bosses are worth 6 regular guys all together. If I did not loot I would be done with each run in less then 10 minutes and hit the instance cap. I don't like armory because Herod and a number of mobs bug out way to often. Looting also pays the repair bill, mounts and training and provides tons of stuff to DE, which is very valuable on my server. I have enough silk to cover the whole of azeroth now but I can use it for first aid and tailoring. I have also made well over 2k gold from BOE blues. It gimps my XP/hour a little but it worth it in the end.