The BATFE letter isn't worthless, but it isn't going to necessarily save you.
1) Any cop dealing with you won't know jack in detail. They'll do what they want and let people with more knowledge and authority sort shit out so long as their ass is covered by probable cause. The converse to this is that they probably won't do anything about it unless you give them a reason to, because they don't know the technical details of the law, if they know the law at all. They either think all evil black rilfes are illegal, or they know some aren't. But that's probably it at best.
2) The NJSP firearms unit is tasked with providing guidelines to departments and prosecutors on the technicalities of the law. They have promulgated the pinkey test. Good or bad.
3) Prosecutors go to court to win cases and make their numbers look good. They won't take a shitty case to court to lose unless they have a grudge or strong marching orders. In theory, when making that determination, they would look at the case they have and make at least a quick consult on technical issues. At that point the actual, specific to this case, opinions of the NJFU will matter a whole lot to them. At that point, the BATFE letter as submitted by your lawyer will also probably matter to them. Once again absent a grudge or strong marching orders, if they both say no it's not, your bill stops being run up there and you move on with your life minus some cash.
4) If it doesn't stop there, you are now going to argue reality vs the law in front of a judge. The law does NOT define a flash suppressor. This means more than likely you will have to down the road of arguing the common understand of the language used in the law. Which will lead to trying to prove that the device does not lessen flash in any meaningful way. The BATFE during the AWB had some unpublished metric for this. Now it does not. This leaves your lawyer and the prosecutor arguing over how much flash it makes vs a bare muzzle, and arguing what meets the common understanding of suppressed. This will likely mean expert testimony. The prosecutor's will be the NJFU guys if they concur it IS one, and possibly additional expert testimony sourced elsewhere. If favorable, one of those expert sources will likely be the manufacturer themselves, and any marketing materials will likely be accepted as the opinion of same in their absence and ignorance of the trial. The prosecutor will not be paying for anything that might help you. IF they have, the entity will just stay home and their results will be secret unless your lawyer discovers their existence and contents. If you are in this position, it means they have been told NO and prosecuted anyway. Don't expect them to abide by the law and willingly disclose stuff they have not made part of their case. SO now you have to pay for experts. Which cost a shit ton. AND you need the judge to not then decide that the common understanding of things is unfavorable to you and your expert.
THAT is what TBill means by the letter being worthless. IT isn't protection from anyone but the BATFE. Vs. state law it is just evidence from an expert source in a situation where you are explicitly asking a judge, who based on precedent thinks you should go fuck yourself, for their opinion on where the threshold of the law is.
Myself, if I didn't care about it being effective as a comp, I'd avoid that risk and just perm-attach a thread protector. IT's safe and gets the job done. It's also cheap.
If I cared about it being effective, I'd buy something that did a good job. And produced as much of a fireball as possible. It would also be absent any verbiage from the manufacturer about flash suppression.