the 4 plunger DB-2, I havent kept up on it in along time since I had decided to go the cummins route, but back in the day when people where making progress at first tinkering with the DS-4 mainly the 5068 it was just thrown out that it would take about 100mm3 to get to 300 rwhp, It doesnt seem like there since then had ever been a whole lot of optimism or interest by many to do any more work on the DS-4 to squeeze out more fuel or even if its possible and as other have stated it is a tempermental IP. A few people said they have acheived high 90's and but I've never read any claims of people getting over 100mm3 with a DS-4.
Just note also that 100m3 would support 300 rwhp, was a therhotical number more than anything else, and was intedend to be a guidline more than a rule.