Yes they will perform better with more air flow, BUT you have to know how to make air flow and still keep the air velocity high. Most racing engine run a very small rpm range, so making power is kinda easy if you know what range you are after. Street cars need to have power at all rpm ranges, thus what you call underpowered is more like well tuned for what it was intended for. Adding high flow heads does nothing if you don't improve the valve train, induction system, and exhaust system. You can't use the improvements if the air can not get into or out of the " air pump " fast enough. Really good heads are those with high flow numbers and really small ports, this keeps the air speed really high. The higher speed air piles up under the valve waiting for the valve to open, this piled up air can actually fill the cylinder over 100% on really good running engines. Really big ports have relatively slow moving air and just can't fill the cylinder fully until the rpm is high enough to over come the port size, this is the low torque at slow rpm you mentioned.
I have a 250 head I worked over using my dad's SuperFlow flow bench. I gained about 20% more air across the valve opening range on the intake side and just a bit more then that on the exhaust side. I added 1.6" exhaust valves from stock 1.5". When I had the head on the truck ( stock 250 ) it ran really good at full throttle and cruise. Try to climb a hill or pass with out going full throttle, it lost all power and shook so bad it broke the shifter mount. My conclusion to this problem was the exhaust flowed more air then the intake, and at part throttle it was pulling the fuel mixture from the intake during valve overlap leaning the engine so bad it couldn't run. The head is headed for the scrap bin this week!
