Flat tappet to solid roller...

If you're not going to want much more lift, then I would avoid a solid roller. Provided you can get decent oil to keep flat tappets alive. I presume you've got that sorted out with your current setup. For one, your lifter bores are probably too large and too varied, they should be bushed, which is another $500 or so. Yes, many people don't; many people also cry about their **** being all blown up after they lose a lifter. Possibly not a common engine machinist setup in Europe, but any shop with a Rottler F60 series or similar CNC should be able to do it with the packaged software.

If you want to stay with Comp and a shelf grind, I would try their hydraulic 21-229-4, which is 251/257 duration at 050, but .564 lift with a 1.5 rocker, and stay with the current 1.6 rockers. I would try to pull weight out of the valvetrain with beehive springs, which should stay stable to 6500rpm. If you can wait a couple months, I'll have a beehive spring 451 built and can confirm or deny where the lifters collapse, rpm wise. Similar cams from Bullet, Hughes etc are also good. Or the Comp MM solid lobes on a custom cam are also an option. Since you're wanting to stay with a reasonable rpm limit, as well as still use it on the street, more duration becomes a point of diminishing returns. S/F....Ken M