If a thief really wants to steal it, then they'll defeat many mods. Heck, even the T-lok could be bypassed by simply crawling under the car and disconnecting the shifter from the trans, hot wire it, pop in drive and away you go. My manual trans setup has 2 1/4" L-shaped brackets welded to the brake and clutch pedal and a deadbold style lock which goes thru the brackets. When locked, the clutch cannot be depressed without applying the brake or vice-versa. Of course, you could always start it in gear and shift without the clutch, but it would be a PITA.
I always thought a neat idea would be a electric mico-lock plumbed into the front AND rear brake circuits with a hidden switch. When you park, you depress the brakes, set the mico-lock and the brakes are now locked until released. It would be a real pain even to winch up onto a rollback.