Warming mats and root development

  Is there a correlation ? I've put the mat back under a selection of the seedlings that seem to be lagging in that department . The ones that were on there are doing well ... of course the ones that are behind were planted about 2 weeks later . Today I moved everything (that I hadn't already)over into 3" square pots . There was a wide variation in root development - and not all can be attributed to the temperature/time in ground , the Beefsteak toms were in the first round planted , were on the mat , and had marginal root development compared to the Roma's (which I transplanted over a week ago) planted at the same time .
  Even with the mat I'm only batting .500 for pepper germination .  Serranos came up pretty quick , I was ready to toss the orange bells when they peeped their little leaves up . The jalapenos and Anaheims still haven't shown and it's been like a month . Note - the anaheim , big bell , orange bell , and jalapeno peppers were all planted in peat pot cells . I've always had lousy luck with those , but I had 'em on hand ... Everything else was in plastic - next year it'll be all new , these are hammered to death . Anybody know of a source for those deep ones that commercial seedlings come in ? Using those I wouldn't have to move seedlings to 3" from the starter cells . Less stress , and happier seedlings .