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Folks have done some weird stuff trying to increase dwell time.
That particular modification would decrease the dwell time, and increase the velocity of the reciprocating parts. By allowing more gas to enter the piston after it has moved backwards you would get increased bolt speed, possibly out running the magazine spring.
 

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Wouldn't be much different then the Sadlak NM version , just in how the gas was directed both ways . Perhaps someone was ahead of the game in experimenting , interesting concept .

View attachment 468641
No, the slotted piston creates a completely different situation.

Note that the slot goes to the back as well as the front, this means gas flows to both the front and back of the piston. The gas at the back gets trapped in the empty volume occupied by the tail and actually resists rearward motion of the piston. The gas flowing to the will balance this retarding force, but notice that it has a longer path and will take a bit longer to get there.

Also, there is the fact that the slot has a much smaller cross sectional area, so the volume of gas allowed to flow will be less and at a lower pressure.
 

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Bear with me... since I can't grasp any benefit to this mod.

I am having trouble trying to figure out why anyone would do this "angled gas port" modification as well.
I am as well.

It seems to me that "mod" would increase the "speed" of the gas piston. ( And why would anyone desire that on a M1A ? ) Since it would flow more gas volume and a longer shot of the gas.
Is it possible, that the added gas volume is causing the gas piston to move so quickly for the first 1/4" , that the whole cyclic speed is seriously out of whack ?
Good possibility.

IE... the gas piston is moving back for that slightly faster time, and the gas system is still under enough full pressure ( bullet still in bore ) and it is leaving the obvious mark ahead of the gas port since its timing is now faster ?
The bullet is gone before the piston has moved 0.05". I have no idea of what causes that extra carbon mark.

With the higher cyclic speed, the case would be under higher chamber pressure while trying to extract. Right ?
Yes, but the main problem is one of acceleration, specifically, trying to accelerate the case from a dead standstill to the high bolt speed almost instantly. The case being slightly inflated doesn't help.

Under what circumstance, would someone desire a higher cyclic speed ? Odd powder burn rates ? Odd bullet weights ?
Extremely dirty conditions, or extremely low port pressure.

I can't imagine the OP's Angled gas port piston would be much like the Sadlak NM gas piston based on the Sadlak description of it.
See my above post.

Quote ...
"If you plan to shoot NM ammo then the groove in the piston is useful. Otherwise, it was not intended for regular ball ammo (not enough power). The groove provides a more consistent pressure against the op rod by bleeding off some of the excess pressure inherent in high-power Match ammo. This results in a slight recoil reduction to help "get back on target" during rapids. It also breaks the vacuum between the piston and the cylinder allowing the shooter to hear the piston slide down to confirm it isn't fouled.

This is a very interesting modification.
I would have recommended a smaller piston port, 5/64 to 3/32 inch maybe?
 
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