How much slower would setting SPCR to 3 make things?
Probably better to use the official API
[How much slower would setting SPCR to 3 make things?](http://How much slower would setting SPCR to 3 make things?)
You can adjust it to 7 different speeds. So
SPI.setClockDivider(SPI_CLOCK_DIV2);
Is probably the default (8MHz) and i would think what you are current using.
SPI.setClockDivider(SPI_CLOCK_DIV4);
would give you 4MHz etc etc.
You do the maths 
but what carries a signal better?
At these slow speeds I doubt it matters.
Rob