BUT!
I say NO if he never plans on a cam, or porting.
We did a single 2.25" to duals exhaust setup on a ~190 RWHP
98 Mustang and only got 2.0 RWHP (that is TWO)
and no perceptible gains over ~ 2-3 TQ in any RPM range.
But then again, our tester then strapped on an ATI 11#
SC kit and needed the duals. But it was an interesting
test, and apparently even around 190 RWHP the single
was adequate, but borderline. Most stock cammed SPI's
are running @ 170 RWHP MAX and maybe 180 RWHP with a tune.
Single exhaust with a good muffler (2.25" size) should be plenty
for a stock car. But most people are not stopping at simple
boltons, and duals are a great idea.