How much stronger are STR shads compared Dexless LUK shads?

Hmmm? Venom is based on LUK and STR equally with DEX as the secondary. The STR they put AP on is won by giving up LUK, the extra STR they get from stuff like zhelms and the likes a LUK shad gets too.
The real reason as to why STR shadowers will beat LUK is simply because a decently scrolled Dragon khanjar blows a godly maple shield out of the water. STR daggers and the blade shields also look a ton better than the LUK ones and the really ugly maple shield.

This. A clean Dragon Khanjar is already an amazing 14-16 attack without scrolling it. You can practically get away with a free godly work glove slapped on your Shadower without even paying more than ~15-20m for a clean one. Paying 15-20m for a 14-16 attack boost? Hell yeah!

Don't forget, the best you can hope for a Maple Shield is about 20 attack. The very fact a Dragon Khanjar is 14-16 attack clean makes it obvious which is better. Giving up 71 STR and probably 10-20 DEX (assuming the level 100-110 LUK daggers are what you're using) is well worth it. Don't forget you can easily make up for the STR with an HT Pendant and a Zakum Helm (it's worth 66 stats to a STR Shadower compared to 44 stats. STR and LUK-wise, it's in essence worth a good 4-5 attack if compared to a MoN). BAM! That's 37 STR right there and also a bonus 17 DEX (assuming you had a MoN to compared to), so all you're really giving up on is 34 STR and ~5-6 attack in order to equip a Dragon Khanjar. You therefore get a net 10 attack boost, the Dragon Khadjar STILL has to be scrolled.

*Besides, who can deny how awesome it looks having a 2nd dagger? Dual wielding Shadower > Shadower with a shield. Therefore, you also get the coolness factor.