Firstly, it is not a dual core machine; the production Samsung/Acers are, the CR48 is single core, hyperthreaded.
I went a similar path with Ubuntu; although I just created a new stateful partition and access the ChromeOS Kernel. This allows me to dual boot when needed. After 8 months, I can tell you, it is not needed. I code and it is just not possible in ChromeOS with the current webapps available.
I have considered switching to full Ubuntu because ChromeOS chose to use a 32bit kernel for whatever reason(compatibility?). However, the processor can't support more than 2GB ram, even with a 64bit OS so I'm open to opinions.
I haven't used the 3G card yet, so I can't speak on it. I am considering swapping it out for a Broadcom HD Decoder so I can watch 720p and hopefully 1080p videos without choppiness.
Did you notice a performance increase when you swapped to a larger HD?
As for battery life, ChromeOS is a little better, but with Ubuntu, I routinely get 7 - 8.5 hours. There are some Ubuntu tweaks for SSD drives that I've done as well. Out of the box Ubuntu is setup for platter drives.
Overall the CR48 has some quirks but is an awesome machine, and I love the design of it more than the production models. I was able to buy an extra one of Craigslist for $150 as a backup. The closest competitor I think is the Macbook air 13" which costs 9x more(!)