The 20% is significant Toby, can ya' loan me a C note?
Yeah, shared servers can be a real bottle neck, that's for certain.
Do you think load times are the reason you've increased sales by 20% ? Since it's only been a few months have you figured in the cost of a redundant server? If not, how much does your backup system set you back? How many domains are you hosting? Do you sell hosting to anyone?
All those things fall into the big picture as far as bottom line costs.