Why would it have to be in a circulair configuration? Granted i dont know a whole lot about RP2.. But does it matter in what way the solar panels are set up? I though they would transfer there power just fine even if there all in strait line.
Yes, they will transfer to neighbouring panels just fine. However there is electrical resistance at work, meaning the longer a distance becomes, the harder it is to move a certain amount of current (amperes) over that distance. Have a look at my blutricity guide. The end result, simplified, is basically that to transfer in a single tick the 2 amps that a solar panel creates per tick, you need a certain potential difference between the panel and a neighbouring conductor. If you have less potential difference available, you cannot transfer the full 2 A, meaning less power (V * A) arrives on the other side.
Thus, if you have a singular long line of 128 panels, you need to pay the "transfer price" so often that you'll only manage to bring a mere fraction of the current all the way to the engine, even with the maximum potential difference of 40 V that the engine can produce (it will not run below 60 V, and the panels won't produce power above 100 V). And keep in mind that you don't have just 2 A to move over 128 steps, oh no. From the first to the second it will be 2 A, but from the second to the third, 4 A, then 6 A, then 8 A and so on as each panel adds its current output.
You can easily determine how much current you can move over what distance with which potential difference with a variant of Ohm's Law: n * Ω = dV * A (number of conducting blocks multiplied by specific resistance of conducting blocks equals potential difference multiplied by current). Solve for n to get the maximum possible distance, solve for dV to get the minimum required voltage difference, or solve for A to get the maximum possible current.
Thus the ideal setup minimzes distance of all panels to the engine, meaning a circular setup. The number of solar panels being 124-128 is a direct result of the engine more and more rising in voltage as you supply more and more power, thereby reducing the possible potential difference you can use to move even more power to it. The state around 24 MJ/t is an equilibirium point where the potential difference allows just enough power to be brought in to maintain that voltage level. Adding more solar panels will not produce anything because their produced power cannot be moved to the engine; the panels will just sit idle.