The class balance (pick rate) was pretty good during the open beta of BF6. I think the synergy bonus is incentive enough. It should just be something small, not something that gives a clear advantage.
Even 10 extra attachment points would convince me to change to a different class just to make my preferred weapon stronger.. making the class system more redundant. As a player i don't really want that. I want a meaningful class system that let me choose class based on the needs of my squad, without forcing me to pick a specific weapon.
I think if you value 10 attachment points over the entire gadget kit of a class, the problem is more in the effectiveness of class gadgets than 10 attachment points being OP.
Either way, one of the benefits of adopting this proposition is that it is very easy to balance. If an additional 20 (or even 10) attachment points is, as you've stated, enough to attract players away from an entire class's kit of a gadgets, then the number of additional attachment points provided by the signature weapon bonus can easily be adjusted. In your case, maybe 5 additional attachment points wouldn't be enough to yank you away from an entire class's kit, though I would wager most people would say even 20 attachment points (about the average price of an extra attachment) wouldn't be too overpowered.
By the time you have spent 100 attachment points on customizing a weapon, all of the high priority weapon attachments should have already been selected and applied to your weapon. Realistically, the extra value you would receive from the additional 20 attachment points would be limited to one of the very last attachments you would select for your weapon, like a match trigger or laser sight. The largest benefit it would offer a player is a little bit more flexibility.
Out of curiosity, what would you individually spend the extra 20 attachment points on that would convince you to switch classes? I implore you to then compare the weapon bonuses this increment of attachment points would offer you to the current signature weapon bonus it would be replacing. I find it shocking that 20 additional attachment points would pull you away from a class, but the current signature weapon bonuses do not.
