traditionally, larger displacement engines had much better low end torque and power deliverly (just look at their torque curves), and could move a heavy car from a standstill faster than a smaller engine, simply due to quicker power application. as multivalve engines made their way into the mainstream, horsepower figures rose, but not proportionally to torque -- torque seemed to get only minor boosts. as a result, a lot of cars with the new multivalve engines saw dramatic increases in speed when the engine was allowed to breathe, but again, only marginal increases were evident off the line. the larger, lower-tech engines still held this domain.
as multivalve design became more prevalent, larger displacement engines began using it (a good early example is the taurus SHO), and they -- due to a good cubic displacement AND better intake technology -- exhibited a flat torque curve not normally associated with a multivalve, high-revving engine. still, big v8's coupled with powerful and intelligently-engineered trannys would beat the smaller, though multivalved, engine in shorter sprints. if the engines were allowed to stretch out a bit, the delta between the two diminshed.
today, we have intake/timing refinements to simple multivalve technology that can drastically change the performance of the engine. variable valve timings that respond to throttle demand help the engine tailor power delivery to the situation at hand -- low-speed, high-throttle situations had one set of valve timing/fuel delivery parameters; open road, higher-speed situations another set. torque curves are flatter than they every have been with multivalve engines due to this accessory technology, and their performance will surpass most reasonably-sized pushrod v8 engines.
of course, most v8s today have the same multi-cam/multivalve heads, so we don't have a smaller-displacement revolution here, we have an industry-wide engine EVolution. it's iterative.
for your particular question, well, it depends on the cars, the transmissions, the drivers, etc. a car making a ton of power and good torque can be crippled by an inefficient tranny or (even worse) a bad driver. i don't believe there is one magic rpm point where a vtec or similar engine will begin to overtake a lower-tech, larger engine, as each multivalve engine has it's intake tech tuned to different flashpoints. my cl-s opens its second intake plenum at ~3800 rpm, and the vtec kicks in around 4400 rpm (i don't know exact numbers...embarrassing), but that's not true for all engines with similar enhancements.
anyhoo, hope this helps.