I checked some stats to clear it up in my feeble mind and here's what I came up with...
During the first nine races of the season Erica qualified lower than 5th only once - that being in Gainesville where she, oddly enough, reached the final and lost to Tom Martino. If you look at it, her performance shows a mild wave (for lack of a better term) of performance over the first nine races, with a couple anomoly's thrown in: She started with back-to-back #2's, the #15 in Gainesville, back-to-back #3's, back-to-back #5's, and #4 & finally a #1. After the #1, and prior to leaving Cagnazzi Racing, is where it gets interesting though: a #10 followed by a DNQ (#18) and then a #6. What happened? (And as was already mentioned, she did make the field in a Dodge in Denver (#13) with a block on the clutch, but you're right, she's been no higher than 17th since.)
Once Tommy Lee got in the seat things didn't get a whole lot better, though he has made the field more often than not: Back-to-back #12's, a DNQ (#18), a #8, a #14 and finally a #23 at Indy.
These stats beg the question... Is she a better driver than Tommy Lee? Could there be that much disparity in one car between two very qualified drivers? OR, did Cagnazzi fall behind in the horsepower department on the Slammers car so Dave could have a better engine since he was in a better position to contend for the championship?
I think she'd have been criticized no matter when she decided to leave Cagnazzi Racing, even though it's always been her desire to race her own team. She's struggling, but I gotta believe they'll get it figured out eventually and she'll be a contender for years to come.