So the bootloader is compiled to operate at just one speed
Yes. 115200bps at 8MHz is actually "not very accurate"; you might be better off staying where you are.
It won't affect anything other than the upload speed.
I have WinAVR, barely know enough to be dangerous, can I use that to compile the bootloader
Yes. Also, the latest optiboot source has some makefile modifications and batch files to help users rebuild it using the tools that come with the Arduino IDE itself. If update the contents of the optiboot directory (.../hardware/arduino/bootloaders/optiboot) with the files from https://github.com/WestfW/Arduino/tree/master/hardware/arduino/bootloaders/optiboot
, you should be able to compile from a "cmd window" with a command like "omake pro8" or similar (see the Readme file.)