I have the 19% and it is a very different beast from the 15%.
With the 19% there is a concern about IAT being a little too high, so it will likely need an intercooler upgrade as well. Since we're in th thick of winter here, I haven't worried about it too much but I will have a revised IC by spring.
The 15% makes for a very well balanced car. The 19% creates a monster and, in some respects, makes the MCS more of a challenge/chore to drive at full-tilt. At WOT with the 19% above 5500rpm your main goal becomes keeping the car on the road.
The reliability jury is still out on the 19% so that's another reason not to do it.
I guess it depends on whether you want awesome acceleration and monster torque steer or a slightly slower ride that's easier to exploit.
YMMV,
Kev