I wouldn't call them exactly lousy, but they're not the fastest. The 300 might give you problems but the 550 should be able to do a big P180 or 182 every 4 or 5 days. The P160s take 2 or 3 on my 400, so figure 5 on the 300.
my 450 is doing a P160 villin at about 45mins a frame, and P180s and P182s are taking 90mins a frame. It's possible that there were one or two larger frames at the begining of the unit, sometimes seems to happen, since 90 mins sounds a little long for a 550 on a 182 or a 300 on a 160.
For non-gromacs work, a K6 Mhz is equal to a P4 Mhz!

I dunno whether that says P4s suck or K6-2s rock.
I think they'll get you about 10-15 points a week between them on average. If the 300 misses many deadlines, you should try switching it to get just
[email protected] work which now counts towards the scores, this is a little easier on the CPU with smaller units and more generous timeouts.
Road Warrior