Not at all.. I think there's enough people like RTech have supplied, fitted and advocated them that they can't be all that bad.. although I am, historically, very anti-cone (not even panel filters get a look in usually, I've only ever had 1 car that has benefitted from one).
Ok, hardly scientific..
On a warm day, normal driving, not sat in traffic, then sat idling outside the house for a few minutes, the thermometer thingy indicated 32 degrees and was climbing quite steadily.
Can't remember if I posted in my thread about fitting the gold reflective tape.. but I did that.. and then..
On a cooler day, a little A-road traffic, after a very spirited drive and sat outside idling for a few minutes, we had 27.5 degrees and slowly climbing.
With nothing to compare it to I don't know whether that's good, bad or ugly.. doesn't seem much noisier with the cone though.. which is ok, my neighbours appreciate it, I'm sure.