I have done ribs on my Bullseye several times with good results too.I changed my offset to +15. When I place my ambient ThermoPro probe close to the Recteq probe there is a very close correlation in temperature. The Recteq is slower to respond to changes, but that's to be expected based on the fuel and the firmware they use. You're not controlling a gas valve so swings are to be expected.
When I place the ThermoPro at any other location I am seeing temps 15 - 20 degrees higher. Again, to be expected as it's not against the side of the grill. I think the temperature at the grate is much closer to the temp the Recteq is displaying when compared to a Weber or other grill thermometer in the top of the lid. On my Weber Smokey Mountain dome temps were 15 - 50 degrees lower than the temps at the grate on low and slow cooks.
I have done a chuck roast for 6 - 7 hours at 225, meat loaf at LO to 375 degrees and ribs for 5 hours, and have to say that Low and Slow does work on the Bullseye.