looking at it if there was no tank, how would you achieve consistent pressure on the output supply line? an unloader can't do that, the unloaders purpose is to relieve pressure between the pump and tank after the pump has shut off so the compressor can restart while not under previous pressure. I suppose one could be rigged to be a pressure regulation mechanism but I don't know how well that would work.
the compressor is positive displacement, so without some large enough buffer tank in relation to demand the output psi would skyrocket, more so the larger the compressor output is to the demand size, the air has to go somewhere otherwise psi climbs... rapidly... until something gets overloaded... but the compressor connecting rod, overheat melt the piston ring, kill the oil, and so on. Not to mention air delivery performance regarding pulsation and heat and moisture.
I don't think you can get around larger cfm compressor output = larger buffer tank otherwise performance suffers in terms of the compressor kicking on and off more often and rapidly changing psi on the output side. but you would have to factor in the cfm's and utilization pressures in relation to the cutoff output pressure... tool use at 90psi with an output tank max psi of 150 where the compressor would cycle however often running output [tank] psi between 100-150. And the cycle time is calculate-able... cfm {minute} and PV = PV where V = cubic feet. so u should do that math... or start posting those numbers here.
