Hi,
I believe that the speed of the 2TB hard drive would be exactly the same as the 1TB hard drive.
I'm just curious, but why would you want to upgrade the size of the hard drive in your computer anyway? The 2TB hard drive (even the 1TB) could easily hold all your files. And with regards to the SSD, it is just holding your Operating System, which takes up about 15GB. So you would still have plenty of room left on the SSD.
With regards to the internal hard drive upgrade, I'm more than positive it voids your warranty, so do this tweaking off your own back! Same goes for the SSD I'm sure. If you can get an Apple Approved Service Centre to do the work for you, happy days! It should void your warranty because it is classified as "unauthorised tweaking." Just letting you know in advance!
But just be careful tweaking inside the iMac. However, saying that, changing the hard drive is a very easy process, same goes for the SSD. (BUT IT MORE THAN LIKELY VOIDS YOUR WARRANTY!) If you really want to know if it will void your warranty, you should email Apple personally. If you get an answer back saying it's fine, then go ahead, and if the computer should ever become faulty, at least you have proof of the go ahead!
To answer your question, the only user changeable part of the iMac Apple intends is the RAM because it is so easy and this doesn't void your warranty. Just be sure to check that the RAM is compatible with the iMac. Also, perhaps you should order standard RAM off Apple and upgrade outside, as the RAM from Apple's website is slightly dearer than where you can get it elsewhere. However, saying that, if you wish to have it done professionally and know your getting the correct RAM for your computer, I would go for the Apple RAM out of peace of mind.
Hope this helps you to decide what way to go.
Chris.