At the risk of angering a lot of people on here I wish to put forward the view that 99% of the increase in performance people believe they get on ART is just placebo at this time.
The ART build on the G3 is the ART build that came with android 4.4.2 - this is now very outdated and in no way reflects the power of ART on the Android L. There are various articles and youtube videos showing that there are virtually no differences between dalvik and ART on android 4.4.2 and indeed most top developers recommended (and still do!) that you don't use ART until android L.
In terms of real world experience, I have had my D855 since release day, I spent one week on Dalvik, then one week on ART, I was amazed by the performance increase (I tweeted about it a lot!) then I switched back to Dalvik for the last 4 days and I have had no drop in performance.
Anyone who has spent a lengthy period on ART I implore you to switch back to Dalvik for a few days, you will be surprised by how little the differences actually are.
I've been an advocate of ART since 4.4 KitKat. On my nexus 5, ART makes multitasking faster than when using Dalvik. I also see a little less stutter when scrolling through pages with a lot of 'post' type elements. This isn't a night and day comparison, but it is definitely noticeable on my end. You can say it's a placebo, but in my eyes, there is a micro-stutter on Dalvik when pressing the multitasking button as well as when switching to another app.
I'm not saying you're wrong and demanding that I'm right. It's just been my experience. I switch apps and multitask a lot, so it's very noticeable for me. I don't see any benchmark improvements, I don't see any increase in battery life, but I do see a difference on my Nexus 5.
I'll give the G3 a week or so on Dalvik before I make the switch though. It depend on how often I see this UI lag that everyone has been talking about. I'll be getting my G3 sometime today. Hopefully before lunch
I should note that I don't sacrifice anything when using ART. All of my apps work on ART just like they do on Dalvik. So it's not like I'm losing anything by using ART instead of Dalvik(on my N5)