My experience is that, yes, it takes longer to cook meat in a cast iron grill pan than in a plain skillet, because (as you say) there is less surface contact between meat and pan. I haven't measured precisely, but I'd say to expect up to double the cooking time, but a lot depends on temperature, whether you use a lid at any point in the cooking process, etc.
This may be heretical to some folks, but personally, I find my cast iron grill pan to be useless for cooking meat. The only thing it's good for is making grill marks, which I don't find useful or necessary. I suppose if you're someone who likes the George Foreman Grill mentality of squeezing fat (and juices) out of meat, you could do the same thing on a grill pan with a grill press or spatula, but I have no desire to do such a thing.
Actual grills work from the radiant heat coming from coals/burners to cook the food, as well as heat conveyed by convection from the air surrounding the meat. Since you don't get much radiant heat in even a cast iron grill pan at high stovetop temperatures, you don't get the same effect on the outside of the meat. And you don't get the same air circulation in a grill pan as you would on a grill, so the convection heating doesn't work the same either.
If you want to approximate actual grilling in the kitchen, I suppose the best option is to broil (which gives you the radiant heat) and then sear very briefly in the grill pan to make those grill marks, which wouldn't take much longer than cooking in a regular skillet on the stovetop in total time.