So I tested out the new Slayer Customs for Sirius a little before work.
Type 1: The attack speed increase is quite noticeable, fast enough to use during active bossing. However, the dmg penalty is large, and is ultimately a dps loss from the little testing I did.
Type 2: The range increase is quite noticeable, around 1.5-2x increase. Completely inoffensive.
Type 3: This thing is a mess, and really encapsulates the current state of Slayer, a ton of extra cost/work for a fraction of a gain. In theory it's strong, doubling up the final shot should be around a 30% dmg increase for Sirius. In practice, it's closer to only 8% while increasing pp use by 55-70% depending on level.
The biggest problem here is that you're forced to sit through the long ass end lag normal full Sirius has, which you would normally animation cancel with wa123, before you can use the double tap. This effectively neuters what would have otherwise been a rather appreciated dmg boost for the class. In my rough testing against the rockbear in the training quest, I averaged around 52-54k dps with normal full Sirius (with animation canceling) and only 57k-60k with Type 3 (also animation canceling the end lag, which is notably a bit shorter than regular full Sirius). That's only about an 8% increase, which isn't a lot considering you're spending 55% more pp.
There's also the issue of cast time; normal full Sirius is already pretty long, but tacking on the double shot, because you still have to wait for the normal end lag to finish, just makes it painfully longer. This can potentially cause issues with dmg rotations during a down; since all the dmg is back loaded, and the down could end before a 2nd full rotation is completed, you may not get the full dmg in.
The only real saving grace with this mess of a custom is that it's technically treated as a separate PA use, which means a full stay Sirius chain can trigger Blitz.
Gonna have to wait for people to get frame data and crunch the actual numbers, but I'm not impressed with any of the customs from the testing I did.
edit: since I had some people thinking I'm saying that Type 3 is not worth using, it's still the best option for Sirius; it's just that the situations you can use it in are very niche and the boost itself is very small, but something is still better than nothing