IF it had that much back pressure it would do 1 of
2 things at higher rpm.. 1.. it would blow the exhaust
off the car(at any one of the joints).. or 2.. it wouldnt
make the rpm... its making some rpm (I dont know how many
rpm he is turning) so at idle/just off idle he shouldnt
see any issues at those rpms... he HAS to put a gauge on
it to see what his back pressure TRULY IS and work from
there.. when I was testing all sorts of exhaust I did see
a system blow the pipe off during a test.. needless to say
we didnt use that system.... the nice little inward roll on
the exhaust tip(on the chrome tips) was a 3hp loss with just
that inward roll on the tip(thats back pressure due to flow
loss).. most people would never think that would be a issue
EDIT
to check the idea of the mixture going lean.. one way
to test that.. pull the exhaust off(open it) and run
it with a A/F meter... On my Rampage I went with elec
dumps just so I didnt pull the exhaust but yet reduce
the back pressure