It can still be a problem, but probably not enough to drop your mileage to 8 mpg. Dang, I think ours is bad in the winter when we do short trips and only get 14.
The catalytic converter and muffler slow down the airflow some and make the whole system work well. You can muck with it some by putting high flow (low resistance) muffler/cat on it, and the O2 sensor should compensate, but I don't really know what the range is that it can make up for.
On a chainsaw, one trick to get more power is to "port" it, basically adding more venting to the muffler, thus less resistance. Then you adjust the carb to pump out a little more gas, juicing it for a little more power. That's all good, unless you have a non-pro relatively recent saw that has limiters on the carb adjustments. I never did get my two smaller saws (stihl and echo) running right after adding just a small amount of extra venting to them, even after overriding the limiters, but I have a couple Huskies as my go-to small and big saws. It's something to play with this winter, and at least one of them now needs a new fuel line, so it's hard to know if that was the reason I couldn't get it dialed in, or if it died in the last few years while sitting unused.
The short of it is, lacking a cat will affect things, but I don't really know if it affects things small enough that the sensors can get the engine to override/utilize the change.
Hopefully today I'll weld in my replacement muffler. This last one only made it 6 years, but it was the bottom of the line. The exhaust tool set you can use from AutoZone is pretty cool.
