Some people have had success using shorter-cage derailleurs with replacement (longer) B-tension screws (any M3 [?] bolt will do). If it's a short-cage 105, this might work if the 30 causes trouble initially. The short cage can cause chain length issues with the big/big and small/small gears, but the jump in chain wrap is small enough that this shouldn't be a problem. If replacing the chain, add an extra link to allow for the bigger cogs in back, and absolutely avoid small/small (chain tension will be too low).

However, the RD is probably a mid/long-cage 105 because the stock cassette has a 28 (OP can correct me if the OEM cassette was smaller), and should take a 30 without problems. It *might* even take a 32.