First off you should knkow that Payne is made by Carrier, so if you hate your Payne what makes you think you will love your Carrier. Just askin.
Second, the load calc said 4 ton. Will the duct handle 4 ton? Does thier bid include duct renovation?
Third as asked before, Did your present unit keep you comfortable? Have you done your own measurments to see if the first co. measured properly, or did they round everything up? This can make a difference on the load, and you did question the sq. footage they came up with. If they included the loft is there proper sized duct going to the loft?
I can't say as I blame the company for not wanting to give you the full report on the load calc, but I would have thought they would have shown you it.