+1 to AT will not have any effect on Exocrines, they still won't be used but the points of every other Bio-Cannon armed creature will need to rise.

Exocrines at 3 for 175pts is tempting, direct fire support within swarms. I think 1 brood may start to sneak into a fair few lists. I don't think +1AT and 3 for 175pts is viable together.

It's a case of do we really want to adjust the points across a wide variety of units just to get a Bio-Cannon boost. The general feel of the Tyranid list involves direct shooting almost always as a combined move (Advance/Double) to get into engage range. If the creatures work why change them?