The diesels were rated at a certain horsepower at a certain RPM limit. By ignoring the limit and bypassing the governors it was possible to run the engines faster than they were safely rated and so produce more than 100% rated power. For an unknown length of time at the expense of an unknown amount of serviceable or possibly non-serviceable damage.
That is the source of the extra power. The diesels did not connect to the drive shafts at all. They strictly supplied power to the batteries and/or electric motors, which were all that powered the sub.
Cutting down the fairwaters was for visibility on the surface. There was very little difference in displacement because the fairwater filled with water when the sub submerged. If anything, the irregular shape of the resulting conning tower would have added more drag to compensate for the missing weight of the steel for very little change in boat handling.
When you reduce cavitation you generally increase thrust. That may have had a little effect. But the fact that the stated advantage was to cut down on noise kind of implies that speed gains were minimal.
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