jg03Rincon,
Your dealer is talking BS about the "tranny computer" being messed up by non-stock tire sizes. The only effect will be that the speedo will read slower than you are actually doing because it calculates the speed based on the speed of rotation of the output shaft from the gearbox based on having 25" tires. With 26" tires you travel 3.14" further with every rotation of the wheel than you would with 25" tires, therefore for the same rotation speed of the output shaft you will travel further (faster) on 26"ers. In practice, when the speedo reads 60mph you would actually be doing 62.4mph with 26" wheels fitted.
Also, the engine will produce no less power, but you will lose some acceleration as a 26" wheel requires 4% more effort to rotate it than a 25" wheel.
The long and the short of it is a slight drop in acceleration and a small increase in top speed - but the differences is very small. The biggest differences will be in ground clearance and handling !