I suspect that even if you were to wear a lounge suit, you'll be likely the best dressed person at the event sans robes.
Dhaller, I have a hard time with the term "lounge suit" as well but used it as the OP had for consistency. Actually I think that is the first time I ever wrote/typed the words;-)Your RCIA advisor will go over specifics with you prior to the event, but basically a suit (I can't bring myself to call it a "lounge suit") or sportcoat and trousers; afterwards you'll wear a white robe over it, so you can color-coordinate accordingly. Remember, respectful and humble, not "look at me!" or fabulous.
(Mine was easy! I wore a Christening gown...)