I'm just throwing this idea out there and would like any comments as to how the system could be improved. This is coming from the idea used by the Corvette Z06 which has a butterfly valve which opens the 3rd and 4th exhaust pipes (downstream of the muffler) when a certain backpressure level (and may some other measured things) are reached.

Now, since spending that much money to develop something that complicated or buying an e-cutout isn't feasible to me, I thought of a possible passive solution. Take a normal cutout, where you have to remove the wingnuts to get it off, install longer bolts and use springs between the cover plate and the wingnuts (you'll probably want to replace that with a normal nut and a lock nut behind it to stop it from backing off).

Now for the mathy stuff. You need to guess (or measure) at which backpressure you want the plate to open. Based on the open area of the cutout pipe, you can calculate the maximum spring preload you'll need to make sure the cutout doesn't open until that backpressure is reached (backpressure x area = force). Now, if you want the plate to open more agressively, you'll need a longer weaker spring (longer so you can get the necessary preload). If you want the plate to open less agressively, you'll need a shorter stronger spring.

This brings us to the next topic: damping. When that plate opens, the system will lose backpressure and the plate will want to slam shut. You'll want to devise some type of system which will add friction as the system shuts but not add too much friction as the system opens.

Random info about backpressure: http://www.thrashercharged.com/tech_htm/exhaust.shtm