1. - I've talked with the Diesel tech and service advisor at the local dealer I deal with and they said no problem with the warranty. They will actually do the gear install.
2. - You are correct that the rear sterling is a 10.5" ring gear. The install isn't difficult, but does require that the carrier is changed to accommodate 3.91 and up gears. The 3.73 carrier only handles 3.73 and down gears. The carriers are only about $80. The install front and rear parts and labor is $1300.
I had 35's with 3.73s and gas mileage was ok, but pulling the 40' toyhauler really caused the truck to shift a lot from 4th to 3rd
Upgraded to 4.10s and gas mileage was hardly affected (theoretically should have been worse), but pulled much better.
Lifted the truck to 8" with 37s with 4.10s and now I am back to square one.
My thoughts overall, if you are looking for gas mileage improvements don't waste the money. If you are towing absolutely do it. Just my 2cents.
2001 F350, 7.3L PSD, 4x4, CC, LB (71k mi), AFE Stage II CAI, 4" DP, 4" Jardine exhaust, PHP Tuner, Mag-Hytec Diff & Trans Pan, 6.0 Trans Cooler, Amp Research Steps.
SOLD (Glad I did) 2008 F450 Lariat, 6.4L PSD, 4x4, CC, 4.30s, (Job 1) , S&B Intake, Banks Six-Gun & SpeedBrake w/IQ, Banks Monster exhaust (DPF-back), Amp Research Steps, Custom RetrofitSource HID 5000k Projectors & fog lights, Train Horn. Still working on it...
SOLD (but wish I didn't) 2001 F250, 7.3L PSD, 4x4, CC, LB (139k mi), HX & pre-tank mods, AFE Magnum/Amsoil Air Filter, 3.5" DP, 4" Magnaflow exhaust, DPTuner F5, Autometer Z-series guages, MAP Sensor Line with Relief Valve, 8" Fabtech Coilover/4-Link conversion lift, Dirt Logic 4.0 & 2.25s, 37" Toyos on 20x9 ProComp 6001s, 4.10s, Firestone Air Bags, AMSoiled (Bypass filter), Mag-Hytec Diff & Trans Pan, TC Manual Lockup Sw., 6.0 Trans Cooler, Amp Research Steps. SPDiesel Exhaust Brake.