If the NSA grabbed your laptop, you've already lost. For instance, they could replace all input and output devices (keyboard, mouse, screen, audio, etc) with ones that not only log everything you do, but also allow them to remotely control your machine as if they were physically present. They could then pretend the laptop was opened (by falsifying the hall effect sensor which detects the lid state), power it on (by forging a press of the power button), log into your account (by replaying the password they logged earlier), and do anything they wanted, as if they were you. They could even use the camera to detect when you looked away for a second while logged into the laptop, and quickly do some input, bypassing any extra validation (like fingerprints or a smartcard) logging into your user account might have required. No need to modify or even touch the boot chain and storage.
If the NSA grabbed your laptop, you've already lost. For instance, they could replace all input and output devices (keyboard, mouse, screen, audio, etc) with ones that not only log everything you do, but also allow them to remotely control your machine as if they were physically present. They could then pretend the laptop was opened (by falsifying the hall effect sensor which detects the lid state), power it on (by forging a press of the power button), log into your account (by replaying the password they logged earlier), and do anything they wanted, as if they were you. They could even use the camera to detect when you looked away for a second while logged into the laptop, and quickly do some input, bypassing any extra validation (like fingerprints or a smartcard) logging into your user account might have required. No need to modify or even touch the boot chain and storage.