Hello
Why would you not believe it runs at 16MHz? 
If you are using an original arduino you can see by yourself by looking at the quartz next to the processor.
It should say S16B00K7 the 16 means 16MHz
in terms of floating point speed it all depends on the quality of the software implementation as the processor doesn't have a floating point coprocessor
we are using the one that comes with avr-gcc so you should check if there is any indication on the avr-libc website
massimo