I use the pickup messages this way some times, but much more often I look at the time as I pick it up. Also I frequently have estimate by subtracting a few seconds if I don't get a chance to look at the time until after the pickup.
I too have looked for a way to increase this duration for the timestamps. I think it can be done by a custom HUD, but I'm not sure if this would be considered cheating or not. I think that perhaps it is a borderline situation.
you should be happy to see the time and the item you picked up...because your opponent doesnt see it all..he has to time by a guess/his last pick/sound when you picked it...
still, it'd be ideal if it can be left on until the next item is picked up