The gun system used on the Cayuse and Kiowa was the M27 system. It had the M134 mounted (a great deal of info is out there about the variants). I never personally saw rocket tubes mounted on the Vietnam era OH-6A. I have seen pictures of such arrangements. Most of this was protype or developed when the 6 was sent to special ops. I have seen the XM8 system-M129 40 mm grenade launcher mounted on basically the same mount. Somewhere online, I have seen pictures of this and the earlier Hughes M75.
The OH-6 had a derated engine due to transmission limitations. I feel that it had sufficient power for what it was designed to do. In a hot, high humidity, adverse density altitude environments, it held its own. Even in the ARNG, in the south, we ofen operated in a less than ideal DA environment. A good book to read is "LOACH' by Wayne Mutza. It contains a lot of good factual info about the Cayuse.
Never personally saw a standard Army 205 body outfitted with a gun system like those pictured. I understand that there were protypes developed and again, seen pictures of VNAF and U.S. field developed and modified long bodies with a variety of armaments. Some seem feasible, some not so-airframe problems, etc.
Spent cases created some problems occassionally, however, some of the equipment solutions: catch bags, deflectors, etc. didn't always work as designed-there are designers and there are builders. I have seen field expedient wire shields on M16 and M21 systems on gunships and behind some door gun stations. Most of that was a long time ago. What I saw after Vietnam were basically standard issue pieces of armament mounted by the book.