I wouldn't worry too much about leakdown figures. A little crust on the valve head (as might be dislodged when removing the spark plug) can throw the leakdown test way off. The shortcoming of leakdowns is that they are conducted in a static condition, so they tend to exaggerate things like imperfections on the valve face, while missing important things like scored cylinder walls. (Valve face condition is very important, of course, but leakdowns tend to exaggerate small imperfections that have no consequence in the operation of a passenger car engine.) I was taught that the leakdown is used to pinpoint problems brought to light in a compression test, before tearing down the engine. In my opinion, the leakdown test acquired glamour because the racers and German car companies use it (not that there's anything unworthy about this group) and it uses more expensive equipment. If you've got 150 psi in all four cylinders in a compression test, I would look elsewhere to explain your low power output. Are your ports or valve heads badly carboned up? Incidentally, I believe that the number 1 (front) cylider in Volvo fours wears fastest because it runs coolest. (It's nearest the inlet from the radiator.)