SLK World Member
Join Date: May 2007
Other Toys: '06 Exige, '65 Ghia
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There is no guarantee. By increasing the RPMs you are, by definition increasing wear and tear. However, any power upgrade, even if done my the factory themselves, has this "feature". What you want to know is whether this increased wear will cause the car to fail sooner than it was designed. A pulley upgrade that adds this little power to a motor this well built, if installed properly is very unlikely to cause an owner any problems. However, you should be sure that your motor is in good working order before doing so.
As for not buying a new belt, you will see very quickly that you will need a new belt. By reducing the size of the pulley, you are reducing the length that the belt must span to go around all of the various pulleys. If you put the old belt on, which would now be too long, you will have a bunch of slack. This will cause certain items in the engine to not rotate as they should. That's a pretty big problem.
As noted earlier, increased pressure begets increased heat. Improving the cooling of the intake charge (intercooler) will remove some of this heat and add some power (a couple of hp).