Hi, currently I have a fractal define r6 case, have 2x140 in and 1x140 out, I open the front panel whenever I'm gaming. Am thinking of changing to a fractal meshify 2 XL case so that I can then have 3x140 in & 1x140 out, for maybe better gpu temp?  I'm thinking the extra 140 intake fan will direct more aiflow to the gpu.  Of course with the XL case being larger I'm not sure how this will affect temps.
 
Any thoughts? Thank you.

OOh I didnt realize that the regular meshify 2 can accomodate 3x140 for input. I will go with that.  And I will try doing an extra 140 on top for output, as you suggest, thanks! Will also trade out the stock fans for arctic bionix p140's for the extra static pressure to pull air through the case's air filter.

You should avoid opening the side panel unnecessarily, as dust can easily make it's way in.
 
If you create a positive air pressure inside (more intake fans than exhaust ones) dust will be less present. If you also have a couple of top fans to evacuate the excess heat, it's even better.
 
As Rmorse pointed out, mesh panels are better for airflow, which is responsible for cooling the system.

So another question. I have some artic bionix P140's (140mm static pressure) fans and some lian-li 140mm uni-fans (which I think are airflow fans). I'm working on upgrading my friend's pre-built pc so I bough some of these fans.  Which do you guys think I should use as intakes, considering that the new case has a mesh front and air filter.  I'm leaning towards the static pressure fans since they move more air. (I'm using all air, no liquid for cooling)
 
Thanks again!

the more air, the better. But make sure you create positive pressure, so have 1-2 exhaust fans.