The phono stage built into the 8000a was built down to the price and it shows. In particular the MC performance is very poor. The MM performance is better if just a bit lacklustre. If you're considering changing because of the MM performance, then this is a difficult choice. An external phono stage will be better but not nbecessarily by a huge amount as this depends on the condition of the amp which is getting on a bit now. However, if you're trying to improve MC performance, a decent external phono stage will improve things dramatically.
So, in summary, if MM there will be a few gains but not outstanding. If MC, there will be significant gains. That said, if you're considering changing amplifier, there are lots around which will take an 8000a to the cleaners for sensible money, and some have good built-in phono stages.