the drive sprocket and/or bar tip sprocket could be worn out. that will goober your adjustment I bet...Also if it doesnt oil correctly it will drag and cause excess friction and make it seem underpowered. then once the chain has been excessively heated, it will never take a good sharpening and last very long, again, making it not cut well and giving the appearance the saw is underpowered.
I bet you have cheaper repairable problems not necessitating a new saw. Although I understand just wanting a new saw.
The only advice I have on purchasing a new one is, Id rather have a used pro grade saw in good shape over a new, but homeowner class, saw, given the same exact money spent.