M-20 Pilot flew into buildups at FL240 while trying to out-climb and avoid convective weather.

Narrative

On an IFR flight as I was approaching a line of clouds with some early buildups; I asked for an amended clearance with a 90 degree turn to the left to try to circumvent the buildups. The Controller instead told me that there was a gap up ahead that should allow safe passage to my destination and cleared me to a VOR and then to my destination. I checked the course and it actually looked like there was a large gap; so I accepted the clearance. I had to get cleared higher several times in order to stay above the clouds. As I was about to enter the gap between the clouds; the controller suddenly called and said that there was heavy precipitation straight ahead as well as on both sides. I told him that the gap was clear; and I could see clear blue sky all the way through. At that moment; the clouds below the gap seemed to rise up and engulf me. For about a minute or so; I was subjected to severe turbulence and occasional ice pellet showers. No ice was accumulated on the airframe; and the engine did not quit. I was able to maintain wings level and did not lose much altitude. During that time; I obtained a low manifold pressure warning and saw 9 inches indication on the manifold pressure gauge. As I came out of the cloud; I told the controller that I needed to descend as soon as possible because of the loss of manifold pressure and was cleared to do so. After descending to around 7000 FT; the manifold pressure returned to normal as did all other engine parameters. I proceeded to land at a local airport without incident. I have noticed no further engine issues in subsequent flights. In this situation; one of my errors was to trust the controller's radar more than my eyes. I was in VMC observing the weather build up ahead; and I asked for a routing away from it that made sense with what I was observing out the window. Even though I was actually given an unsolicited clearance to a temptingly quicker way around the buildups; I should have stuck to my guns. Another error was to misjudge the tops of the clouds ahead of me. In retrospect; the air was rising; moist; and unstable; and even though I was heading for what looked like an easy pass between the building clouds; I was unable to recognize the rapidity with which the clouds were rising directly in front of me.