Ultimately whether you need the 80 degree LP is dependent upon a bunch of factors as there is no one-sized fits all approach.
The 80 degree LP is designed to slow the opening of the action and retard the amount of rearward bolt travel speed in the MP5K firearm in order to reduce the ROF in full auto and prevent damage to the receiver.
The K receiver is not as robust as the full size receiver when it comes to being suppressed or fed a diet of hot ammo.
- You have less receiver length so the gun has to mitigate the recoil from the rearward bolt travel in ~25% less distance (the full size has ~4.5" of bolt travel vs~ 3.5" in the K)
- Most K stocks go inside the receiver vs. the full size that goes around the outside of the receiver like a clamshell providing additional support on the rear of the gun
- The K rear pins don't have a welded in rear cross bushing supporting the pin like the full size does so the recoil force when the bolt bottoms out is transmitted directly the thin sheet metal holes.
- To the Ks benefit HK added the second upper rear pin holes and the supporting weldments at the rear.
Factors that come into play as to whether a specific MP5K needs a 80 degree LP include:
- Ammo (grain weight and powder charge)
- Suppressor and the amount of back pressure the can generates.
- How fresh/strong is your recoil spring
- Are you running a 9mm hammer spring or a rifle hammer spring.
- Bolt gap to a limited extent.
If you have a low backpressure suppressor, running lightly loaded 115gr ammo, with a new recoil spring, and a rifle hammer spring you may find that your K won't reliably cycle with an 80 degree LP.
Conversely if you are running steady diet of 147gr +P self defense ammo, with a high back pressure suppressor, on a gun with thousand of rounds down the pipe with a well broken in recoil and 9mm hammer spring and buffer you may find out the hard way you damaged your receiver via roller dents or wallowed out rear take down pin holes over time as well as a highly increased ROF in full auto.
Ultimately its probably the safe approach to test your specific mix of ammo, suppressor, and host gun with the 80 degree LP if you plan to run the gun with the suppressor on a regular basis. If the gun functions with your combination and doesn't have FTE issues and exhibits a healthy ejection pattern, than stick with the 80 degree LP. If the gun functions with an 80 degree LP that will save wear and tear on your receiver long term especially as your springs and buffer wear in.
If you put an 80 degree LP in and your combination of gun/ammo experiences FTEs or really weak ejection pattern than go back to the 100 degree LP as you can damage the extractor spring if you experience a bunch of FTEs plus its no fun to have a gun that doesn't function right.
Its the same as the AR15 world as there is no given recipe as to whether you need to run a heavy H3 buffer and/or an adjustable gas block just because you add a suppressor. A lot has to do with the suppressor, ammo, gas port hole size, etc.
The full size MP5 is not a sensitive to suppressor use given its longer/stronger receiver construction but it certainly doesn't hurt to run a 80 degree LP in the full size if you are running a steady diet of hot or heavy grain weight ammo along with a high back pressure suppressor in order to keep the bolt from battering the stock and back of the receiver.
In terms of bolt gap change, the poster above is correct that bolt gap will usually change to some degree if you change the locking piece out. That said if you have a German gun (SP5K) and change the German 100 degree for an 80 degree the change is usually minimal and it won't necessitate changing the rollers. You may see a 0.001 to 0.003 change if you stick with all German parts and in my experience going down in angle usually result in the bolt gap going up.
If you buy a US made LP than its more of a mixed bag in terms of bolt gap change in my experience. Either way its prudent to check the bolt gap after changing the locking piece out but it usually a non-issue sticking with all german parts.
Hope this helps.