Stock Rocker Ratios

Agreed. The mechanical 273 rockers can and have been found to be off. You can have them re-worked and corrected when they get checked for there accuracy. Hughes engines can do this for you.

It has also been found that some after market companies have rockers not living up to the advertised ratio as well.
Do be aware that even if the ratio is dead balls on accurate, you will loose lift due to the pushrod angles. (AKA - geometry)

Roller rockers are pricey for most any manufacturer except a Chevy, so, in a major of speaking, a lot depends on you wallet thickness and need or want. Try a search @ summitracing.com and price check them. (And in another plug for) Hughesengines.com also has really nice rockers in 1.5-&-1.6 for the LA heads.

HP wise, there only a few HP. But performance wise, as in set up, accurate ratios, getting that intended lift, properly actuation the valves, there a good purchase IMO. Are they needed? No. But they do help insure that your getting the cam action from the cam that you purchased based on spec's. So getting every last bit of what you purchased, from the cam, a good accurate ratio and set up of the rocker arm is needed.