The 915, and the 901, are not "just an ancient design" or a" balky recalcitrant beast." It was designed by engineers from the company with the most significant history of motorsports of the 20th Century. Properly maintained it is more than capable of doing a fine job.
Fix the gearbox, fix your technique, or fix both. The G50 in my '88 Carrera had more in common with the 1980 Toyota SR5 pickup that I learned to drive a stick in (and heel-toe, for that matter, it was a hell of a vehicle) than the 901's in both of my cars.
If you need any further convincing I humbly submit the below video of Walter Rohrl shifting a 901 box. Turn the sound down and enjoy.
http://www.youtube.com/watch?v=U0u_sWeqDwc&mode=related&search=