i did the same thing you did. bought the rapid, loved the way it rode, then got road bike envy. in the end i got a 2nd bike but I can tell you that Microshift has shifters that are much less than sora and BETTER than sora that will work on your setup although no matter what set of shifters you switch too, you will need to change your front derailleur unless you decide to use friction on the front.

Also, don't be afraid to buy 9 speed shifters, you can make them work with your 8 speed setup or you can just buy a cheap 9speed casette to put on your rear wheel and convert to drop bars with 9 speeds. you can probably sell your old shifters and casette to recoup money on the swap.