will the stock tailshaft work with the t50? i'm pretty sure the t50 is longer than a k50.
I'm just of the opinion if you are planning future upgrades (even some really large cams or quads) then simply use the aftermarket ECU from the beginning. why do something twice?
However, if its going to be standard forever then the stock ECU would be better.
My smallport 4age in my ke20 is quick (i had mine on the road, engineered for 10 months) but its not stupid quick. Now the turbo conversion is almost finished, it has saved me money having the haltech already there.