I did mine eighteen months ago, no issues all positive, yes improved power but I have L77 on e85 (see below), I used Yella Terra Ultralite 1.85. I would probably do Mace 1.9 if I had the chance again.
Following read is a wall of experience - at first the +30rwkw leap from stock with tune and OTR, plus sales testimonials attributing larger diameter bolt-ons to better performance, lead me to believe larger diameter pipes in and out of the engine will result in increased air flow = equals more power = faster car. That was way too simplistic:
The engine’s stock peak HP RPM is 5300, with HP being a product of RPM, limited by valve events (specifically 0.480" lift). Opening the engine to replace the camshaft in the middle is effective but somewhat drastic.
By comparison increasing lift and duration simply by removing the rocker cover reveals the the upper valve train, replace the stock items with high ratio rockers, high performance valve springs, and moly rods, is quick, less expensive, simple.
Higher ratio rockers immediately gives you revised event duration same as LS3, and lift as per LS2. The result is stock manners, AFM, while slightly increasing volumetric flow and the engine’s redline through calculated control of components.
Flex fuel responds better to more trapped Ve as you can tune for higher cylinder pressure without knock at lower RPM (higher torque per ci). So dialing in timing with an e85, more lift and colder plugs delivers more torque across the board, while revised events increase volumetric flow and redline, which results in more HP up top.
However although the fuel, timing, valvetrain is capable of much more power using these events, the heads want better flow than is available through the stock manifold. This is across the board, not just up top.
In fact the stock CSA of the runners are way too big to deliver air at the velocity required for the 6.0 to realise potential torque. The manifold is a compromise in the first place, adding more lift and slightly more duration spikes intake velocity without the corresponding increase in flow.
So while 12-20rwkW better than before at high RPM with HSV extractors, mandrel bend and hi-flow cats, the main restriction becomes the intake manifold and to a lesser extent the throttle body. Also while the MAF can be scaled it is not as reliable as SD only at this state of tune due to frequency resolution. MAFless or bigger MAF.
For the GenIV intake to deliver more flow with less velocity you need to derestrict and port the plenum and tumble the charge through the runners using a radius bar. This induced tumble slows velocity and increases CFM, and is race proven to promote better combustion in the chamber across the range. In my car it added 15rwkW and
So yes, rockers are worth it on our engine (better with flex fuel), plus modded HSV headers, plus modded intake manifold and throttle body. With this combo it is possible to increase Trapped Volumetric Efficiency without spending or changing a lot.
Personally I spent too much to discover all this, but am confident the car was one of the more powerful VEs using bolt-ons. The next worthwhile stage FWIW has been to further increase potential torque through really long Tri-Ys and x-pipe, and multiplying it earlier using a shot peened 3.27 TrueTrac diff. Bottom line is a change in handling and character 0-100 in 4.8, massive in-gear accel, and excellent highway kickdown.
)