I think you mean m/min not m/s!

On my X-axis I got both ends of the screw machined to match BK12 blocks, but for different reasons. I don't think this increased the price much if at all. It would probably help more if you went for custom end machining and blocks which increase the spacing of the bearings on the 'not fixed' end. Do you need more than 6m/min though? That's a fairly respectable speed, certainly plenty for cutting metals and most things efficiently.