At the point the exhaust vavalve opens, the cylinder pressure is still 70PSI to 120PSI. That exhaust exits past the barely open exhaust valve at supersonic velocity for a fraction of a moment until the valve opens more and the pressure has bled off. This high pressure and velocity in a tuned exhaust eventually causes a negative cylinder pressure about the time the intake valve opens.
With a tuned intake there can be a slight pressure increase as the intake valve lifts off the seat. This is very minor as compared to the exhaust when considering a NA engine. Basically the best we can consider in a NA intake manifold is atmospheric oressure of 14.7 PSI. Therefore opening the intake as much as possible is generally beneficial.
As per David Vizard, the flow is pretty much area ruled until the valve lift gets to or exceeds 0.25D. At 0.25D of the valve the area of the valve head and the area of the circumference of the seat at said lift are the same. Once you get the valve open farther than 0.25D lift, the port takes over as the controlling factor instead of the seat area between the head and valve. With this a 1.9" intake valve is at 0.25D lift at 0.475". A cam that only allows 0.425" to 0.44" valve lift remains restricted in flow by the seat, not the port flow capability. With this example of intake valve diameter, a cam profile combined with a rocker ratio to give intake valve lift of 0.500" to 0.510" gets the valve out of the way to allow the port to limit the flow. Now this is not much in degrees of crank rotation or milliseconds, but does improve the engine performance noticably. You have not affected cam duration by more than a couple of degrees at 0.100" to 0.200" lift and not changed overlap at all with a higher ratio rocker, so idle quality and low speed driving are not effected.
Getting the exhaust seats cut or ground to 40° greatly aids blowdown and the strength of the exhaust pulse tuning. Using the 40° seats on the street allows you to use less exhaust timing and possibly a single pattern cam for more low speed torque, which is what low speed driving around town entails.
For the race crowd that may be ready to come unglued on my comment, yes at high lift 50° and 55° seats do provide higher flow. Another advantage in a high RPM engine is the steep valve angle allows the valve to wedge into the seat better, preventing valve bounce and thus promoting better seal.
A street engine that normally putz's around town and a race engine are two different animals for design criteria.
