With the bypass filter installed, the soot load will still increase, but the size of soot particle agglomeration will not escalate quite as quickly. But eventually, even bypass filters cannot keep up and the sump needs to be change.
Most operators try to hold the soot around 3% to perhaps 4%. You're nearing the condemantion level IMO. You might be able to get another 250 hours out of it, but there would be a need to watch it closely from this point forward.
You'll have to consider the cost of a UOA versus the cost of sump/filter change. At some point, it becomes cost effective to OCI rather than test every xxx hours. But, the upside to this approach over the long term is that if you can develop steady data, you can actually fine tune the UOA cycles and OCIs and get the max useage for least cost of analysis.