The rMB specs are obviously not mind blowing, but in the end, it really comes down to the type and size of projects you're working on. As a project grows, you'll surely notice some slowdown at one point in time, but it's hard to predict whether this slowdown will be a show stopper or not.
I think the best way to go at it is to simply get one and stress test it as best as you can ( e.g.: throw your biggest project at it, or check out some hefty open source project and see how it handles ). You have 2 weeks of playing around at your disposal, you can either keep it or just give it back... the only inconvenience is the wait for getting your money back if you do decide to give it back.
Seeing that you'd be downgrading from a 15" rMBP, expect to be somewhat underwhelmed ( assuming that the 15" rMBP is currently not a huge overkill for your needs ).
