I hope I do not confuse myself here ! When I switched from flat tappet to roller on my 351W I was advised the roller cam would allow me to get a bit more radical specs within the given cam's power band to 6000-6200rpm with less compromise. I went from .520 to .576 lift and actually dropped 100rpm at idle . Also, I was advised the roller cam reduced friction allowing the motor to rev smoother, faster and last longer.
Of course, making the swap did cost about $450 because my 351W needed the spider plate across the
oil valley to better secure the new roller lifters plus new roller lifters, rockers and cam. Some newer blocks may not require the spider plate install. My decison was made during a rebuild so rollers/heads/cam/lifters were already a given as a required expense.
If I had a great running flat tappet set up with roller rockers only and simply wanted a bigger/better cam I'd probably just invest $100 for a better flat tappet cam and slide it in. Or consider going up in rockers (if you have clearance) from 1.6 to 1.7 ratio. Otherwise, from the ground up I never regretted going to a full roller cam set up and would do it again.