High EGT's and low RPM

The best that you could do would be to run a borescope up the tailpipe.

This will only let you view the exit chamber.

As can be seen in the link above of the 2-stroke muffler, the internals can be quite convoluted with 3 or even 4 internal chambers that overlap and reverse the flow.

Unless you know what the internal construction of your particular muffler is, the best you could do would be to look in the openings and if any chamber looks marginal, assume the ones you cannot see are even worse.

It would be a rare and poor design that only has one baffle separating the exhaust pipes from the tailpipe.

And to close this out, here is the happy ending.  I ordered a new muffler system (have to order all of the exhaust headers to make it fit right) and have it installed.  It is a LOT beefier than the original. They clearly learned some things about the mufflers on the iS in the last 5 years or so.  The problem was that the exhaust pipe stuck our laterally an inch or so more than the original requiring some significant cutting on the cowling to make it fit.  But it works and in the long run I feel a lot better about this one.  I did screw up an EGT sensor in the process and have to order a new one.  The lane A light started flashing before I even started the engine.  EGT 3 immediately went to overheat when I did start it.  

Thanks for all the help from everyone.  Hopefully this thread will help some other hapless pilot who can't figure out why his RPM's are low in the future.  

We are all happy that you found the solution.  Thanks for the update!

The sharp inside corners that you made when cutting the cowling are just asking for fractures to start at those corners.

Round holes and/or larger radius corners are required to reduce fracturing.  Heads Up! 

Thanks, I will round those out.

Well done Paul!