Most of my Jet2 flights have been on the 733 and leg room hasn't been a problem, the problem with 4+ flights is you need more personal space, i have not yet flown on a Jet2 757 so can't comment.
You can't build a transatlantic program on adhoc seasonal charter flights at Jet2 current frequency around xmas charters, it works only because its low season and aircraft and crews are available, operating multi weekly frequencies require reliable fuel efficient aircraft, a tech problem down line will wipe out a weeks worth of profit
Whilst some operators can use USA/Canada flying to cover winter low season, you need modern kit to do it, unless and until Jet2 think outside the UK box they are stuck with seasonal flying, next year 2016/2017 will be good for 737 rated pilots, Norwegian, Ryanair will need hundreds of crews and will be offering UK bases + maybe Monarch 737's order might happen, i expect my income to increase by at least 20% in 2016 off the back of training and basic increases after few flat years