Mike, I have just finished (well almost) a RSH from Highlevel with CSB's... on such a small loco and building it to P4 with the bespoke gearbox I really havn't seen any difference in running compared to my Blackhawthorn which is compensated. So I would say yes to CSBing the front axle but leave the back axle fixed.... Biggest hassle with most of these things is the clearance behind the cross head to the crank pin. I would say all doable but the Split chassis due to the clearances might be a bit too far.