Hi
Is it LM1117 which overheats on mega board? It has only 800mA output current capability and it overheats quite easily. Can you measure how much does tft's backlight led consumes, I think that's the reason why mega's regulator overheats. I've similar 5" tft and it consumes about 250mA and it's enought to heat regulator already so hot that finger doesn't like it.. 
edit: Another question: Is your 5" tft also jumped for 3.3V like this 7" is ?
TommiP