What does the TT weigh loaded. My last TT had an 8400 GVW and weighed 6200lbs. New 5'er has an 11,000 GVW and weighs 6500lbs. You don't add the tongue or pin weight to the loaded weight as those are resting on the truck/SUV.
Nothing wrong with up grading to E rated Maxxis in a 15". They have a CC of 2830lbs @80 psi. If your rims can handle the psi it would be cheaper. Most very 15" 6 lug aluminum wheel that is put on TT's of recent years is rated for 2830lbs. Check here and see if you can find your rim.
http://www.tredittire.com/Aluminum-Wheels-C10.aspxIf your wanting to get into an LT tire then yes upgrading to a 16" would do the trick. IMO 17.5 is way overkill. And do you have the room between or above for the tires?