You can basically do what you like - the pipework from the external
stopcock onwards is your responsibility, so you can change it if you
need to. Not sure how it would improve things though.

Because when other devices draw water there will be less effect to the
boiler if is teed off closer to the rising main. ...I think.

The difference from teeing off either side of a full bore stopcock would
probably not even be measurable, let alone significant. OK, there might be a
difference if you are knocking out a large section of 15mm pipework, but I
don't get the impression that this is the case.

Christian.