Thanks for pointing out the obvious 
Long story ....
But the gist is constraints of resources and time to test the apps and monitoring thoroughly !
So we want to migrate to ASE 16 with minimum surprises.
And hopefully we will get to test the threaded mode before we upgrade it in place.
BTW : Feedback from SAP so far is that there is a heap memory pool fragmentation occurring. We are investigating further to find the cause(s).
Avinash