Just going to roller rockers is not going to free up any measurable HP. If you are talking about roller rockers with a higher ratio, like 1.75:1 compared to the stock 1.6:1 rockers, then you may get a few extra HP (probably not 15, though). For $ spent, roller rockers are not a big HP gainer. I just bought a set of 1.75:1 standard rockers for my 300 for $46.

Roller rockers are used in high performance engines with high valve lift mainly because they reduce side forces on the valve stem and valve guides.