Have you ever had a trans apart? It's a lot more difficult then just opening it up, replacing the synchros, and bolting it back together. If you want it to last for more then one thousand miles, that is.

For grinds, I like to replace the trans oil a couple times, allowing about a thousand miles between changes. If it's just as bad, it's time for a used or rebuilt trans. D-series engines don't put out neary enough power to justify a performance trans, mind you when you say "performance" I think multi-thousand dollar dog boxes.