Hello,
I am using the High-Low Tech Arduino 1.6 AtTiny Core files and it annoyed me that they don't support the internal 16MHz PLL clock.
So I changed the boards.txt file a bit and there is now an option for both the Tiny45 and 85 to run on 16MHz.
The Brown-Out detection is enabled to trigger at 2.7V
To install it just replace the existing attiny/avr/boards.txt file with the one I attached and it will work.
I tested it with the AtTiny45 as well as the AtTiny85.
Uh... I must have made a mistake there. I used an online calculator for the fuse bits. I might have accidentally checked it.
I updated the file in the post.