I know this is old (found by searching), but I wanted to add something to it.
There is absolutely NO way back pressure can affect pressure applied to the exhaust cam lobe. It is not physically possible. Why? Here's an explanation:
Say you built up 150 PSI in the exhaust. Where is that pressure applied? To the BACK side of the valve head where the stem is, not the part in the cylinder. In all actuality, it relieves pressure from the cam by applying force to the back of the valve to help open it. If you force enough pressure into the exhaust port, it will slightly open the valve, relieving pressure, thus causing the valve to close again due to the loss in pressure (much like a BOV).
Think about this, too. If you have no back pressure at all, when the exhaust valve is opening and the the piston begins to make it's way back up, the velocity will be higher because there's no apposing force. You have x amount of PSI inside the cylinder and 0 outside the cylinder, needing y amount of force to move the gasses out of the cylinder at z velocity. Now, if you have back pressure that measures 50% less than cylinder pressure, your velocity will decrease and the force needed will be greater. However, the pressure witnessed by the valve is greater when they're NO back pressure, as it's relative to pressure differences. The valve sees 100% of the pressure difference. If your cylinder pressure and exhaust back pressure are equal, there is ZERO difference, thus the valve moves as if you're turning it over by hand (no fire, no real cylinder pressure, no back pressure, just like opening in closing at atmosphere). Think of a one-way check valve. If the pressures are the same on either side, the valve basically flops around until pressure changes on either side, at which point the valve either opens or closes.
Now, a vehicle will have trouble running with a clogged cat because; A) it's harder for the motor to push out the exhaust gasses, and B) residual pressure in the cylinder may cause a slight 'push' into the intake when the intake valve opens. These two combined cause a motor to lose efficiency.
Now move along...lol!