I realize this is an older post, but if anyone is wondering about the answer to these questions as it would apply to HTC THunderbolt with ICS (yes, ICS finally came to us Bolt Heads), I can tell you my opinion.
I installed ICS on my Bolt (2/2013) and think ICS is wonderful upgrade from our original software. HOWEVER, the lag time between apps and when loading anything or switching form one app to another or from email to messages or to open and surf web... OMG, the lag time is awfully slooooooowwwwwwwwwww, as in sometimes four to seven seconds just to get a new app opened or to switch from one to the other.
For comparison purposes, I went to a Verizon store and played with the Razr Maxx HD *(dual core, with more 250 more RAM and .5 more processor, but again dual core processor). Side by side, doing exactly the same actions (with same apps, etc)... the Razr Maxx had essentially zero (as in maybe microsecond delay) switching between apps and operations. It was embarrassingly obvious that the long awaited ICS for Thunderbolt was simply not a good idea. I walked out of the store knowing that my phone is the equivalent of an old 486 PC. It is just not the right phone set up (single core without enough RAM and not enough processing speed) to handle today's operating systems.
The Bolt was an awesome phone and if I could go back to the old software, I would. However, that is like saying i would rather use my 10 year old laptop for gaming than the brand spanking new one with all the "stuff" optimized for today's software and games.
Simply stated, there is no comparison of single core vs. dual core, particularly in light of the fact that *most* dual core phones are new enough that they also have critical other upgrades (processor and RAM).
So, for anyone wondering if it is worth upgrading your Bolt (or other single core) type phone to a dual core phone, to enjoy/take advantage of ICS or JB... the answer is yes, yes, yes. This is ESPECIALLY so when looking at the fact that as of March 12 you can get the Razr HD for a PENNY or the Razr MAXX HD for a hundred bucks through Amazon!
Just a reminder: Droids Rule, iPhones Drool.