I have the same tiller extender on my M15. Installed it about 4 or 5 years ago. Works great! Allows you to move forward and sit in the front of the cockpit. I remember having the same concern regarding drilling the 7/16" hole, but I did and it has been just fine. I drilled mine about 1'' back from the tip of tiller.

On another note; my twist lock action quit last year. Sent 2 e-mails to Forespar (1 recently), but haven't yet heard back. Took matters into my own hands, it was either service it or replace it! Decided to give servicing it a try. Extended the tiller fully and using a tubing cutter I cut off about 1 1/2 inches of the larger end. This gave me access to the internal workings. Found the twist-lock action is accomplished by a split nylon ring. Mine was stuck! Freed it and stretched it a little and the twist-lock is working good again. A permanent fix would be to replace the split nylon ring. While I am thinking of it will e-mail Forespar again to see if the part is available. To my knowledge, there is no other way to service the unit.