Does Mandrel bent pipes really make a difference?

It is a fair difference.
I don't know exactly how it stacks up in your combo though.
2.5" mandrel pipes actually outflow 3" pinch bent pipes, for example.
Alot of folks get together a bunch of mandrel bends then cut/weld them into the shapes they need. That is something you still may be able to do in your shop...
Dave

I know that import guys make a huge deal out of mandrel bends - but when your motor is a 4 banger every little bit helps.

A guy I know owns a custom exhaust shop, and he tells me that 3" pipe is really to much for a naturally aspirated small block (you lose to much back-pressure.) So I wonder if a mandrel bent 2.5" might have similar issues? Or if that is even true?

The whole point is fairly moot point on my '68. The only bent pipe I have is the one that clears the axle - of course it is a heck of a bend!

As for sound, there will probably be a different sound, but I don't know how so. Shouldn't lose volume though. If I had to GUESS, this is just a WAG (Wild @$$3D Guess) but I'd say it would be less of a growl to the sound.

Mandrel bends make a lot of difference. If you are going to spend the money on a new exhaust then you might as well get the best system for your money. You can piece one together with parts available form Pro-Motorsports if you like. I have made several systems with their mandrel bent tailpipes and the results were always fantastic. Get aluminumized pipe whatever you do so the system lasts for a very long time.