There is no difference between the 899 and the 1199 exhaust.
The only difference between all the kits are as follows:
899 - slipon, upmap key for 899 ECU
1199 RACE - slipon, upmap key for RACE maps on ECU
1199 EVO - full system (headers and slipon), upmap key for EVO maps
So basically this is the full Termignoni system and the Akrapovic fuel mapping.
I would have gone with the front exit Termignoni full system instead which likely flows more similarly to the Akrapovic than the legacy Termignoni.
The 899 does not have a full system specifically designed for it because the headers flow well in relation to the engine output and slipon exhaust. The improvement would be minimal and the only benefit comes from installing the Akrapovic titanium system which lowers the weight.
In all, the stock exhaust on the Panigale flows extremely well so you will not see HP or torque gains like you do on a Yamaha, for example. Smoothing out the torque curve is the most prominent gain you will notice.
2014 Panigale 899 - Red
Termi Exhaust, Rotobox Wheels, NRC Fender Eliminator, Rizoma Rearsets and Reservoirs, Yoyodyne Slipper Clutch, Ducabike Clear Clutch Cover, CRG CF Levers, DP Fuel Cap, 'R' Seat Cover and Decal Kit
R&G Radiator Guard, Fork and Tank Sliders
2015 BMW S1000RR - Red
HP Wheels, Yoshimura R-55 LE Exhaust, Competition Werks Fender Eliminator and Integrated Tail Light,
Rizoma LED Indicators, Fuel Cap, and RRC Levers, Bonamici Rearsets, R&G Sliders and Rear Spools