I'm bowing in a scratch league that splits the season into thirds. Each third has a set pattern. Currently, we are on a 43' flat high volume pattern. In a few weeks we will switch to a 35' flat high volume pattern.
I am a med speed&rev tweener med to low tilt, my release is inside and behind the ball
My current arsenal is:
RotoGrip Haywire, pin in, CG kicked way out, OTB finish
900Global Chemical X, pin above ring, CG slightly out, OTB finish
Sysmic Photon LMB, pin above ring, CG in, OTB finish
Storm HyRoad, pin in, CG in, polished finish
Brunswick Meele Jab, pin above ring, CG in, OTB finish

Looking for suggestions on what to look at next with the 35' sport pattern in mind. I'm not loyal to any manufacture.

I think there are getting to be quite a few balls out there that might work.  Original IQ Tour solid would be one of the first I would look at.  Arson Low flare solid, and Viral Solid might also be balls to look at.  A lot depends on how you want to try to play the lanes.  I like to play up the boards from outside 7 board on the shorter patterns and it suits my game.  Thus my recommendation of the low flare tamer solids.  Also, I am not familiar enough with anything in your current arsenal, other than the Hyroad, to rule any of them out.  Basically something with an arc look similar to what you get from urethane.  You might even have luck with urethane on this pattern if you can deal with carrydown. 

Maybe I'm just being ignorant here, but why would a benchmark ball need to be different for a flat shot(no matter the length) than a THS?

I am new to flat patterns, so just looking for suggestions on what ball to start the night with.

Right now on the 43' pattern, I start with my Chemical X and then move to either the Haywire or LMB as the lanes change, usually its the Haywire

One of the big things on short and flat patterns I look for is something with a smooth reaction shape.  This is because with having that extra backend, you will usually get a much sharper move. 

The suggestion about playing up the boards and using solids/low flare covers mentioned previously is one I really like.  It keeps things simple, helps you keep everything more controlled and helps you score better.  In addition, pay attention to your hand release.  If you usually have more side rotation, try to limit it by staying behind the ball more.  This also helps limit the change of direction and will get the ball to roll more than being continuous.

Are you looking at a certain company if you're drilling something new?

Personally, I like my Grip It with a 2 Inch pin to pap and low hole. Can't get too left to right with it, but it reads the lanes fabulously and I can tell when I have to ball change to something with more motion off the spot.

Open to all suggestions from any brand. I am looking to add to my arsenal