On the PI in a 100 watt Marshall with 50uf the amp will have less backbone that with the standard 100uf filtering. Some of the early plexi Marshalls had 64uf on the PI but they ended up going to 100uf sometime in late 68 and stayed with that filtering on all 100 watt models.
You can try 50uf and see if you like it but I preferred 100uf versus 64uf in my 68 plexi.