LionUWF, you make the assumption that the extra 10% will make a difference for everyone. You also fail to add in the HCI effects on number of attacks for a Yew-enhanced weapon.
According to the devs, SSI effectively counts as stamina points, for determining weapon speed. However, fractions above each multiple of 30 are wasted if you don't have the matching stamina to make up for it.
So, your choice in SSI amounts to whether that extra 10% from ASH will raise your swing speed.
125 Stamina + 40 SSI swings at the SAME SPEED as 125 Stamina + 30 SSI (165 vs. 155 = both round down to 150).
If you have equipment to go to 140 stamina, then 40 SSI will give you a faster swing speed than 30 SSI - but only while you are at full stamina.
If your equipment gives you 150 stamina, then 30 SSI with full stamina will be the same as 40% with 140-150 stamina; in effect, just giving you a slight buffer for lost stamina from movements or hits.
As I dont have a suit that would push Max Stamina + SSI over 180 with a 40% SSI weapon, I typically stick with 30% SSI yew-enhanced weapons on my archer, for the extra Hit Chance (more so than the DI).
So, really, it comes down to the choice between Ash & Yew being a niche thing. You need to know the template and suit of the person the bow is for, and then decide whether having 10% more SSI will have any effect in the long run. More people will opt for the SSI out of feelings, more than practicality, though it will in some cases let you tough out one hit and not lose swing speed (but then, that's what Divine Fury and refresh pots are for). For others, SSI will have a real effect. But, there's also a few groups where SSI over 30 doesn't really matter, or they require a different number (ex. a PvM mage using a sc mage weapon bow as backup while letting their mana passively regenerate between spell dumps, who might only have 40 stamina - and therefore could probably get away with an amount of 20-30% SSI on their weapon - especially when you consider that most of the intensity of the bow will be in SC, FC 1, and mage weapon, and they will want to leave some room for imbuing mana leech, before enhancing)