"Is there a specific speed for a pellet for bench rest long range shooting"
Yes.
And it is the speed that your rifle groups best at, any number anyone gives you only applies to their rifle, grab 5 off the shelf same model, serial numbers in order and they may not all want to shoot at the same fps for the BEST groups.
Test, test, test.
Generally under 880fps , nothing wrong with going slower as long as groups hold - and they should in most cases- . Sometimes, depending on how you go about getting there lower fps/fpe really helps, a rilfe with less mass moving inside ( lighter striker/hammer , shorter stroke, ... ) can help calm a rifle down.
BR is an odd thing, it's not getting say 25 straight bull's that wins it's the one miss that looses. Naturally you work with whatever you have but if shopping for a BR rig I personally would be tempted to use a very calm sedate 12fpe rig.
Write everything down and let us know what your rifle ends up liking best, always a good starting point for others that wy.
john