Congress designed the revolving fund with the purpose of spending any funds in excess of costs to help the rest of government be more efficient and effective, as those funds don't naturally return to the US Treasury as they would in other parts of government.
In terms of 18F's performance in both regards, the capital start up funding we've received from the fund has been extremely well spent IMHO, but of course I'm totally biased. I'd look at what we've produced with that $ and let the work speak for itself, especially given how much govt would normally spend on this much information technology: https://github.com/18f
That also only captures things which have an impact in software - we're still working on the best way to represent the fiscal or performance impact of our consulting work.
All that said, of course we can always be more efficient ourselves, and I think the next few years of fiscal reports will show that.