What do people consider an acceptable difference in lap times in the name of safety?
Interesting question, not sure if lap time is the deciding factor. It is more on the lines of if the cars move in similar fashions - straight line speed vrs apex speeds and if the other drivers get all pissy about mix class racing and will either race you cleanly or give you room to get by.

First wheel to wheel race I ever did I got passed by a Trans-am/GT1 car on the front straight at Bridgehampton. Closing speed was 50mph+. But we all played well together and no issues. OTOH, I've been yelled at by other class drivers and slammed on this forum because I didn't give up my fight for position to allow someone else to get through with lap times w/in a second of each other.