MacOS – Will the Mac run slow if I update it to Mavericks

You could add the 1600 MHz memory for your MacBook Pro even though it's designed for a slower memory (1066 MHz) - it ought to work, but you'd be better off buying from a place that accepts returns. Here's some relevant information from Crucial on memory speeds (emphasis mine):

Memory is designed to be backward-compatible, so generally speaking, you can safely add faster memory to a computer that was designed to run slower memory. However, your system will operate at the speed of the slowest memory module.

In "DDR3-1600 PC3-12800"

• "DDR3" is the type (and must be the same as what your computer has)
• "1600" is the data transfer rate per second specified in millions (the technical term is "clock frequency") - "1600" here means it supports up to 1600 million transfers per second - in your case, it will be slowed down to 1066 million transfers per second since that's what your Mac is designed for
• "PC3-12800" is the name used in the memory industry - 12800 indicates a peak transfer rate of 12800 MB/s (of course, the actual performance in your case would be brought down to 8533 MB/s, matching PC3-8500).

Are you trying to clean install OS X Mavericks? If you are, you should download a copy of Mavericks, then restore it onto a external drive, restart your mac onto that install drive, go to Utilities, then to 'Disk Utility' then erase your hard drive. Next, install Mavericks.

If you tried this step and booted with the install drive, I do not know if I can help you.