Horizontal component of first stage velocity will be under Mach 6 or under 2,000 meters/sec as Elon Musk stated previously (in a Popular Mechanics magazine article from 2012) for recovery of his 1st stage. Remember that SpaceX announced F9R wind tunnel tests at NASA-Marshall up through Mach 5. Remember that overcoming gravity losses is ~ 1,500 meters/sec of 1st stage delta V so total 1st stage burn out delta V is ~ 3,500 meters/sec = 2,000 meters/sec horizontal velocity + 1,500 meters/sec gravity losses.
That 15% number is a reduction of orbital payload by 15% (or 30%) and probably not a massive 15% fuel reserve on 1st stage of 60+ tons propellant. A 32 ton propellant reserve is more likely.
Air Force RLV booster studies and Kistler had estimated rocket-back maneuver to launch pad, including bleeding off Mach 4 to 7 in horizontal velocity, to require ~ 3,000 meters/sec delta V (according to the cited AF RLV booster return study that states 11,000 fps - 1,000 fps = 10,000 fps = 3,000 m/s as delta V needed for return to launch pad). This is closer to 32 tons propellant reserved for the 3,000 m/s delta V needed for the 1st stage to rocket back to the original launch pad.
Elon Musk has already said that F9R landing legs weight are less than Tesla Model S weight of 2.1 tons and that mass fraction of F9R 1st stage is under 4%. You should assume 18-tons including legs in empty weight and ~ 32-tons fuel to bring 1st stage back to pad. They need to be able to throttle Merlin 1D back to 30% thrust (i.e. 20-tons thrust) for a soft landing of an empty 18-ton booster.
Air friction alone can slow booster to subsonic velocities, but it can also tear booster apart. SpaceX has to control reentry and position vehicle using multiple smaller firings so it does not "belly flop" (Elon Musk's words) into the atmosphere.
Use these numbers as your estimates:
418-ton fueled 1st stage with 18-ton empty weight (i.e. 16-tons for 4% mass fraction and 2 tons for the new landing legs) -- http://www.spaceflight101.com/falcon-9-v11.html;
50-ton burn out weight of 1st stage with 32-tons fuel left for rocket-back maneuver;
2nd stage fueled weight of 75-tons, empty weight of 5 tons, and payload weight of 13-tons, for 88-tons total payload + 2nd stage weight carried by 1st stage; and
total F9R rocket weight of 506 tons to place 13-ton payload into LEO and to rocket back 1st stage to launch pad.
I also assume 300-sec average Isp on 1st stage and 340-sec Isp on 2nd stage.
The above estimates fit with the F9R v1.1 weight and performance estimates made by this web site -- http://www.spaceflight101.com/falcon-9-v11.html
The above estimates should work in your calculations for F9R taking a 13-ton payload to LEO and then the F9R 1st stage having the delta V of 3,000 meters/second (obtained from 32-tons reserved propellant) to be able to perform a rocket-back maneuver to the launch pad.
I estimate that it will be less than 3-tons of added weight penalty on the F9R 2nd stage for the propellant, heat shield, (4) Super Draco thrusters, and landing legs needed to recover and reuse the F9R 2nd stage similar to recovery and reuse of the Dragon-2 spacecraft.
This means that a fully reusable F9R that reuses both its 1st and 2nd stages could carry 10-tons to LEO and less than 1-ton to GTO.