OMG! I just saw the price on those things @ tileredi.com
But Shim, you need to look at this as a business decision. The money is spent, there's no way to undo that. So you need to forget about the money spent in the past and figure out what is your best choise going forward from here.
For starters, it doesn't make much sense to me to spend $$$ on a shower pan, then attempt to use it in a wrong way. That just sounds like you're setting yourself up for a $$$$ fix in the future. And as a DIYer myself, I HATE working on something, only to have to undo/redo it again.
So if were me, I would be trying to decide which of the two following courses of action are best all FUTURE things considered:
1. Use the Redi Base and get the job done quicker and easier (but I wind up with a shower smaller than I want).
2. Spend the time (because it sounds like not much money involved) to install a traditional pan and get the size shower I want.
Now one thing that does come to mind that would make option #1 more paletable would be to possibly rethink the orientation of your shower. For example, rather than planning on turning your side to the door, plan on turning your back to the door. Get one of those shower heads on long arms that are designed to direct the water DOWN rather than at an angle. Then it doesn't matter which way you are facing relative to the shower head. Then again, a long arm isn't REQUIRED... nothing says you MUST mount a shower head on the wall (consider mounting it on the ceiling). Of course that has its consequenses too (such as more time between adjusting the temperature a feeling the result).
You can also still use the Redi Base and NOT have the door centered. Place the door at the far edge if water isn't going to easily splash up under the door. You won't gain much (perhaps an inch). But that's still more space with very little effort if your door can be set up so that it looks like it was ment to align with the outside edge of the curb.