I am running a 28 ft boat with twin 454 power. It runs at about 4900 rpms. I would like to drop down to about 4600 rpms, this is the recommended rpm for this motor. It currently has mirage 25 pitch props on it. I would like to try 27 pitch props to see what happens. What is the difference between the mirage & the mirage plus ? What should I expect moving up in pitch ? Can I get more speed this way ? Will this drop my rpms down 300 or 400 rpms ? Any thoughts or comments are greatly appreciated !

Mirage Plus's generally spin about 200rpms less than plain Mirages of the same pitch and diameter. Even though they spin 200rpms less, there is is usually no difference in speed. Basically, they have more cupping and are more efficient. They also get out of the hole and hold better in turns. Sounds like the right solution for what you're trying to do.
A 27" Mirage would lower rpms by about 400rpms as a general rule. Getting them labbed would add 200rpms.