The neoprene shirt worn under the FJ with full-length legs is not bad for water flushing through. Mine both fit snugly.
The flush thru all depends on body shape and fit of the farmer J.
The OP suggested that he was of two different sizes top vs bottom. This is one of the reasons I suggested a pair of neo pants paired with a neo vest. It allows the wearer to fit both the top and the bottom and lower any flush thru.
It also allows the OP to pair with either a pair of neo shorts or neo pants and also different weights of both depending on weather and water temps on any particular day or week or month. more versatility. {also more ability to evolve the set up and add more options as time goes on} YMMV