Max sequential write of an internal 7200rpm HDD is only around 100MB/s I believe, but you'd probably not be talking a great deal more than the speeds you are getting now.
Your USB transfer rate is actually pretty good, I only get about 20MB/s myself.
Some SSDs peak at around 500MB/s, but their true forte is boot times, and small random read and writes.
The HDD slows down, sometimes up to 50 percent when it's near completely full.
1.) Maybe be 2x quicker, probably not worth the hassle
2.) SSD transfer rate is quick, but the HDD write would slow it down anyway to about the same speeds you have now.
3.) USB connector is probably the bottleneck, yes.
Personally, I would not bother messing around with it too much for the speed gains you'd get.
But, thats my personal opinion, you might think otherwise mate