question

This should give a rough idea on output from panels on a flat trailer.

Of course it all depends on where you are in the world, but Effectively it shows that generally speaking when at between 30-40 tilt angle (assuming this is then 0 degrees to the Sun itself) - that you get 100%, and when flat (0 degrees) - you still get around 90 % (keep in mind it is of the rated capacity inclusive of the Panel efficiency and environmental factors, etc.)

As far as 2P plus 2 more - going 2S x 2P will likely give you the best power curve - Serializing gives more hours of energy due to the increased overall Closed Circuit voltage (Vpm) and subsequently the Parallel will maintain the increased Current.

As far as the reduction to max 700 - that is simply based on the Unit being rated to 50A max. so 50A x 14v - therefore 700w max at 12v

Therefore if you go parallel into a 24V battery bank that increases to 50A x 28v - ergo 1400w

So really you would not get much benefit from the extra panels unless you:

a. change to 24V batteries.

b. change to a larger MPPT.

As an additional real world comment, I have a pair(2) of 315W panels that over-produce their rating a bit, and are mounted flat. I routinely see 650 W out of these panels for 1-2 hours around local noon.

So at peak production you'd be throwing away most of the output new panels. However... for much of the rest of the day you'd see higher production with the new panels. Still, seem to me you'd be better off adding to or upgrading your existing MPPT. The controllers aren't that expensive, just add a new 100/50 for the new pair.