You will have diminished efficiency: The First Strike kit comes with a stiffer spring and a different bolt. The bolt requires more pressure to power the round, which required the stiffer spring to return it without damaging the body. This consumes more gas.
I have a T9 with the longbow kit and the FS kit, and was surprised that I could only get about 17-19 shots out of a 12-ie.
Per Tiberius Arms this is normal, and you should plan accordingly when out on the field.
I ran into this issue on my last game, I had 4 shots left in my longbow mag, but no gas to run it.
If you want the higher efficiency you will need to change back to the regular bolt and spring.

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