Won't a radical lopey idle , play hell with idle vacuum and power brake function?
Yes. The questions are
1. Sure a Thumpr can make power. How much MORE power AND DRIVEABILITY would you have if the cam was not sabotaged to create an artificially-horrible idle?
2. How bad can the idle get and not cause functional problems due to low idle vacuum?
3. How are the emissions police going to react to a vehicle that intentionally misfires at low rpm?
4. How much engine life/cylinder wall/ring wear/catalyst life are you willing to sacrifice in order to make the engine misfire at idle?
High-performance engines tend to idle poorly. This is not a
desired trait, it's merely a by-product of a camshaft and cylinder-head porting/sizing optimized for higher-rpm use. Then guys decide that they don't need the high-rpm power band, but they still want their engine to "sound mean".
They'd be better-off to ground one plug wire (or disconnect one sequential port fuel injector) when they drive to the Dairy Queen to impress people they don't know and who do not care. Ground a different plug wire each time, to spread the excess cylinder-wall and ring wear evenly among the cylinders. But the catalyst is going to live a really hard life no matter which cylinder is deactivated.
Optimize the camshaft for the power band that the vehicle will be driven in; and for a pickup truck or SUV, that better be set up for lower-rpm power especially if the cylinder heads, intake system, and exhaust won't support high-rpm power.