The regular GT can come with brembos. It's way more than a sticker package though. The Boss mustangs get some weight reduction (only weighs 3600), adjustable suspension, different cams and intake manifold.
From Fords media site:
What most people don’t realize is that engine stresses increase exponentially as engine speeds rise,” explains Harrison. “So moving up from GT’s 7,000 rshared_pm redline required significant re-engineering of many different parts. Sacrificing reliability and usability over the GT engine was never an option.”
Some of the Boss-specific parts contributing to the Boss 302 V8’s output and durability include:
Revised composite intake system with shorter runners, inspired by Daytona Prototype racing engines, for high-rshared_pm breathing
Forged aluminum pistons and upgraded sinter-forged connecting rods for improved strength, needed for the higher combustion pressures and engine speeds
New high-strength aluminum-alloy cylinder heads with fully CNC-machined ports and chambers for exceptional high-rshared_pm airflow without sacrificing low-speed torque
Lightened valvetrain components to provide excellent dynamic performance up to speeds well above the engine redline
Sodium-filled exhaust valves for improved heat dissipation
Race-specification crankshaft main and rod bearings for higher load capability and improved high-speed durability
5W50 full-synthetic oil with engine oil cooler for improved oil pressure and longer-lasting lubrication during extreme racing conditions
Revised oil pan baffling for improved oil control under racing conditions and during cornering loads greater than 1.0 g
Also the laguna seca models are apparently going to have a "red key", from what I was able to read it accesses a different tune that is more aggresive and allows for a redline beyond the regular boss's 7500rshared_pm.