I do know that I drive torque, while listening to my friends prattle on about horsepower.
Lumpy idle on gassers is because the fuel/air charge isn't in the right place at the right time more or less. You don't have the intake runner velocity at idle to keep the fuel in suspension all the way in to the chamber through compression. If you think about a massive big block with a tunnel ram or something, at idle how much air/fuel mixture is it really pulling from that intake volume? Big lumpy cam with lots of overlap doesn't help either since again there's so little velocity at idle you have a hard time getting any kind of swirl or tumble going on. Remember on a gasser at idle the throttle is almost completely blocked off so there's no air rushing in behind what you're sucking in on the intake stroke either... Overlap doesn't work too well when the pressure in the exhaust is higher than the pressure in the intake! Anyhow the result is by the time the fuel has left the carb, hung out in a huge intake and then barely been sucked in to the engine it's half separated from the air and doesn't burn all that well. Hence why you end up having to run a crazy rich idle on a big block with a massive cam, massive intake, massive carb, etc just to keep enough burnable fuel in the cylinders.Since diesels don't add the fuel till they burn it you don't have any of those problems. It burns when it's injected, and the injection amount and timing is dead consistent.Result is diesels have a nice smooth idle no matter how much you hop them up. A motor ported out to flow like crazy with a monster turbo pushing insane pressure will make gobs of power, but still idle nice and smooth.Not that it has any impact on idle, but you can't really run any overlap on a VW diesel. When the piston is at the top of the exhaust stroke it's almost touching the head - no room to have any overlap between exhaust and intake without putting the valves through the piston!It MIGHT be possible to get a bit more lift than the stock cam has, but I have no idea if it would really help or not. More duration is pretty much out of the question, as are larger valves. Best thing you can do is improve the flow on the head, give yourself a good free flowing exhaust system and a good intercooler - get the exhaust out and as much fresh air in as possible.The great thing about all of the stuff you CAN do though is that your engine will still have great street manners!