One of my characters pulled a mobility outfit last night, and I tried it on just to see what it was like. I kept the inventory open to check what happened to my AC while I tumbled. While tumbling, it went up by 4 (duh), but it stayed up by 4 until I moved around. While standing in one spot after the tumble, I still had +4 AC, even while swinging my weapons. Is this the way it is supposed to work? Was there possibly a bug causing this (it was suggested that it may have been a visual bug, with the inventory AC not reflecting the true AC). These were questions I'd have to answer. If it does work that way, it seems that mobility is much more useful; I have a rogue that typically tumbles into combat, swings a while and tumbles over to someone else, it would be nice having the +4 while I'm doing the swinging.
THE TESTS
First, I checked that the mobility outfit was still having the effect on the character I noticed the effect on. It was; during AND after the tumble (instead of just during, as the feat states), my AC was up by +4 as long as I didn't walk backward or forward. I could pivot and swing my weapons, and shield bash with the mobility boosted AC. It disappered as soon as I walked, (I monitored my AC from the inventory screen).
Next, I sent the mobility outfit to another charcter. Same results.
To check that it wasn't just the item, I rolled a human fighter with Dodge and Mobility, and went through the grotto. Again, occording to the inventory screen, my AC stayed boosted as long as I didn't walk after the tumble. Here, I was able to test a bit with enemies, but nothing conclusive.
Finally, I went back to one of my characters with the mobility outfit and went PvP with one of my guild mates. I had him swing at me after I tumbled (18 AC pre tumble, 22 AC during and post tumble). He sent me a tell with every swing that totalled in the 19-20 range, which missed me. This seems to confirm that the mobility AC boost stays after the tumble itself, as long as you don't move from your spot.
I'm thinking of respecting some feats, but I'd like someone else to test and see if they get the same results. This seems to make the Mobility feat a lot more usefull. With Dodge and Mobility, there is a reliable +5 dodge bonus to AC, as long as you use tumble to position yourself. What do you all think?