Just had the CrossClimate 2's put on last week. I can attest that yes, your mileage will go down. Hopefully it improves but I am not confident it will. We put about 150 miles on them this past weekend traveling back and forth for my son's dek hockey tourney. They do ride nice but, I need to let some air out. NTB aired them up too high, so my mileage will likely get worse. They are rocking at 44 psi cold right now. Door frame states 36 psi. I may split the difference and go with 40 psi all around and see how the ride changes with that.
Had to make a move: current OEM Toyos were at 4/5 32nds and wife was afraid driving in mod. to heavy rain. Traction control was on constantly and she could feel them hydroplaning. Only 17750 miles on the OEMs. Junk.
Cost for NTB as $939 OTD. Called 3 places and couldn't get them (Sam's, Costco and local indy tire shop), was quoted $1,113 OTD at Flynn's tire and called NTB and their price was $1,230 OTD. I asked about their price matching and they came down to $950 over the phone so I made the appt. At the appt., checked in and the store manager checked me in and said he could do even better and got me down to $939 OTD. Sam's, even if they had them, wouldn't be able to get that low.
I just hope after the initial wear in my mileage improves but for now, the ride and handling is much improved and winter handling will be far and above better than what we had previously.