Most likely impossible at this point based on the sheer impossibility of getting the original through the wall.
Also, I come in UNDER 14....which, while close to 15 isnt 15. I could attempt to vent it straight out and see if it helps....how long should I make that run? How does one support that?
If I took the top horizontal run off (temporarily) would that mess anything up? It would drop my EVL down to 8....but it would be venting straight up