I am re-posting a reply I had made a while back regarding this subject as I believe the information may be useful (especially regarding a trans cooler):
I use my FX35 to tow a 16' Arima with a 90HP outboard. It is always fully loaded and I always have 2-3 other adults in the car.
Over all - I love towing with this car!!
There were a few issues - but are now resolved.
Power - NO problem - this thing pulls my boat up a local steep grade much easier and faster than with my previous Toyota Tundra (V6)
Steadiness - OK - I feel the load more than I did in my truck - but not bad - not scary at all
Noise - was bad - the hitch (OEM) rattled like crazy - sounded like it was in the back seat but I bought and installed the Hitch Stabilizer that was in an earlier post - great fix - no problems now
to purchase one (30 bucks with shipping - well worth it and very easy to install)
Gas Mileage - Terrible - do NOT monitor it on your onboard computer - it will just make you crazy (towing is an occasional event for me - so just suck it up)
Comfort - Great - and the back seat passengers are so much happier than they were in my truck
Transmission - BEWARE!!! - after pulling the boat for on 4 trips it was time for service - my mechanic told me my trans fluid was fried. They flushed the fluid and I added a trans cooler.
(I knew better and had planned on adding the trans cooler when I had the hitch installed - unfortunately my mechanic had mistaken the power steering cooler for a trans cooler and so he told me I already had one.)
If you tow anything that has any real weight Ė GET A TRANS COOLER! These things are actually good even if you donít tow as many of these high performance transmissions have problems with over heating. A trans cooler can NEVER cause harm. Cost me about $300 installed.
I had reservations when I bought the FX as I needed it to be my sole tow vehicle and I can now say, without any reservations, this is a great tow vehicle for my size boat