Sentinel Frigates as they are now have a cap of around 19/17/15 for mk1/2/3 and they do 40,000/80,000/120,000 every 3 seconds on normal combat style IIRC, so they CAN toss out a significant amount of damage. The problem I've found with them is that because of their low numbers, they still have a really difficult time of dealing with massive hordes of higher tech AI ships. They tend to be better on defense, or if you use them as a surgical-strike instrument on offense where you pop them in system to blow up a guard post or an AI Eye or something, and then pop them out.