First your list of questions:
1. No difference. (At least there shouldn't be).
2. Yes. Are you not using the composite clipper now? That will boost it quite a bit, but the quality will be MUCH better.
3. Depends on what sound card you buy, and which connector(s) your transmitter has. It's generally not difficult to connect it, there are convertor plugs from/to everything, price is usually around $1-5.
4. YES (if you use the composite clipper). Unless you are using an Omnia.9sg (which has the same clipper/stereo/rds coder as Stereo Tool), you can expect both several dB's of extra loudness and a much clearer sound.
5. Marian is DC coupled, so if you send out a DC offset it will stay there - that means no need to calibrate it. Except for that, many other cards are good too.
6. I would go for a Marian card, either the Marian Trace Alpha (which is PCI - that might be an issue!) or the Marian Seraph AD2.
About the CPU load: The composite clipper will make it higher, but on the CPU's that you're mentioning that won't be a problem at all. For comparison, on my laptop (i7 4710) I can run 7 Stereo Tools with full FM processing simultaneously. (No low latency in that case though