Thinking of exhaust mod

To a degree, yes, combo dependent.
If the engine is low powered, an H will do fine. An X would also be a bennifit, but it's also a cost vs power gain issue and it would be expensive for little gain.

Same idea on very powerful engines. Sometimes, a true dual exhaust with sewer pipe size exhaust pipes is what it needs and performs best with.

The main idea of a X pipe is for scavging. (SP!) If you have stock heads and a single pattern cam, the X will help huge

It's a very combo dependent thing.