My 50K mile 87GN needed the tranny rebuilt right after I put in a TA49 and 2800 stall TC. More power finished it off. A buddy of mind experienced the same after adding more power. It's just a fact of life. You can just run yours until the tranny starts slipping, usually the 1-2 shift. Then a rebuild is $1500-$2500 depending on how bullet proof you want it - race vs. street.
When the time comes, I highly recommend only having it rebuilt by one of the reputable 200-4R builders named in this web site.