Engine got updated for 2015. Both the SRT ( while it was still made ) and scat pack used the same motor. Hence the powered by SRT on the motor and radio splash screen of scat packs. The differences are in the suspension, brakes ( scat packs can upgrade with dynamics package ), wider tires etc...
There is no such thing as a SRT scat pack. The scat pack is a R/T and is badged that way.
The Scat Pack performance kits are available in three levels, with the Scat Pack 1 kit starting at $2195 for both Challenger and Charger models. This level adds a cold-air intake, a Mopar cat-back exhaust, and a new engine chip that adds 18 hp and 18 lb-ft of torque to the 5.7-liter Hemi V-8, which as stock makes 375 hp in the Challenger and 370 hp in the Charger.
The Scat Pack 2 kit, which costs $1895 extra, builds on the level 1 kit and gains a Mopar performance camshaft kit and a stage 2 performance calibration chip that helps the engine breathe better for an increase of 12 more hp to the stage 1 kit, or 30 hp over stock. Lastly, the Scat Pack 3 kit sits atop the range and costs $4995 in addition to the cost of the level 1 and 2 kits. This level's performance ported cylinder heads, high-flow headers and catalysts, and upgraded engine chip make for an extra 58 hp and 47 lb-ft of torque over the stock V-8 models.
These Scat Pack level 3 Challengers and Chargers come closest to their SRT equivalents, which make 470 hp and 470 lb-ft of torque from their 6.4-liter Hemi V-8 engines. A fully-equipped Scat Pack Charger makes 428 hp and 442 lb-ft of torque, and a Scat Pack stage 3 Challenger makes 433 hp and 457 lb-ft of torque."