There is a really good document on Powerlink called the Networker 7.6 Performance Optimisation Plannng Guide which answers your questions in detail.  I know you are not using Networker 7.6 but this document contains a lot of good information that is valid for all NetWorker versions.  There is also a version for 7.4 if you need information specific to this version.  You can find the Guide at:

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-Bobby

Bobby made a good point leading to that document, you should read that. But here are some fast answers:

1) See quetions #3

2) Yes, you can

3) Check server, client and group parallelism, and also target sessions. A quick look in the Administrator's Guide for these words will bring you a lot of info, and also a lot of things to think about (sometimes to improve backup you'll have to sacrifice restore time, so there's a balance here you'll have to consider)