I'm a low time Mooney pilot but I don't get why people often claim mooneys are hard to slow down. With just a little bit of planning they slow down great. Personally I start slowing pulling power and adding nose up trim 5 to 10 minutes before I arrive at the airport. I arrive on altitude at pattern speed. I never have to use the gear, flaps or speed brakes(don't have them) to slow down. Maybe it's cause I fly a G model, the slowest of the mooneys.
I flew with a guy one time in a J model who would come into the middle of the downwind doing 140 knots, seemed to me that was the hard way to do things. Some planning ahead makes it nice and easy and adds some consistency that will make it less likely that you will gear up. In the end you probably only land a minute later than if you came blazing into the pattern at near cruise speed.
Not an instructor, just personal experience from someone with very little personal experience in a Mooney.