Well I fitted a UK legal Delkevic titanium S-Oval end can on my Hornet today. I am pretty pleased with it to be honest, sounds good, fair bit deeper but not obnoxiously loud. I don't think it makes much if any difference to power but it sounds faster . Original end can was 7kg the new one is 1.45kg.
I've had the Dbl-D's for years on my Shoei & Arai lids.... they do feel more secure.
But now I've gotten used to the latch-type on my Bell....although it can sometimes be a PITA w/ my 5 thumbs on my LH....
I have both, both on Shoei helmets. I prefer using the DD type but the fixing points for the latch type are higher up in the lid and therefore hold it on my noggin more securely, at least that what it feels like. Thankfully ive never had to test either.
I haven't seen a double D helmet in ages. Having said that my last 3 helmets have been Nolans and all have the toothed latch, haven't thought about it for ages either.