Yes large tires like that will cause a large drop in performance. I put 315's on my F-150, then added a gryphon and I was maybe as fast as I was before, but I think I was a little slower. Those tranny temps seem very high for not towing anything, but that will probably be lower with different tires. The body lift might be pretty tough to remove, but I would think the easiest way to do it is to go to a local shop and ask them how much they charge. Also the body lift will be ok with smaller tires, but it will add drag so you will get worse gas mileage with it installed. The other option is to change the gears in the differentials, it is ex*****ve ($1500-$2000 from what I have heard), but it will put the tranny back in the right gearing range, and give you more power to the tires.
On another note, the 315, 265, 285, is just the width of the tire, it has nothing to do with the actual diameter. The difference is going to be in the second number. It is a ratio of the width to the height, so you could have a 265/70R17 and a 275/65R17 and the diameter will be very very similar, or a 275/70R16 would also be close. I have 35"ish tires (they are for sale) that were 315/70R17, and total diameter was 34.5", while yours are total diameter of 34.6".
, In your case where you drive on damp grass I think the wider, 285s, will work better and get you stuck less, but on snow they will be slippier so be careful of that too. Sorry if I got off topic a little, just thought you might want more info than what you asked for lol
