The cowl flap / speed brake provides another mechanism to steepen the approach in addition to a slip.
They are a safety feature to give additional, approach angle control to augment a slip if needed.
Assuming you do it correctly, otherwise it turns what should be an orderly glide to approach into a plummeting spine-splintering slam to the ground. That's assuming you don't pitch the nose over and end up with your engine in your lap.

I'm getting you are set on doing this and I've said all I can as to why there is a better than even chance that it will work out as nicely as you think it will.. If you don't mind my asking, can you PM me your address as I have a release form I would like you to sign before you pursue this any further.