Bk: sorry about the uv not working out but at least you tried and learnt something. Little by little in the midday, low watts and higher hanging height, starting in veg already should be the way forward. Also thinking about the spread and not watts per total area. It might be hard to incorporate uv in a bar fixture as they tend to hang lower than boards.
If you clone: whatever you clone from should have some uv on it before making clones, this should harden them up a bit. And even more so if your doing seed runs, one buddy breads with uv in order to get more potency and better handling, but i think hes off growing for a bit, no posting here or anywhere for a long time but i see he still visits the private forum, in Narnia.
Another thing would be compensating the uv with far red but might be tricky, dont known if it will counter uv damage or just counter some of the uv effect.
Another thing that might be relevant: you need higher uva than uvb or you get damage. I heard 3:1 ratios being thrown around. So maybe you would have the same effect between standard 450 blue; you need to not exceed 450nm/ square feet adding the new uv. This would make sense as this is what the normal sun spectrum look like. But then again also remember that most indoor cultivars/cuts havent been out in the sunshine for many generations. Hard to say.
Catskillze: just an idea i had just now, if you find it hard to get stretch under control with 450nm then try this: offset the timer of the blue so that its on for another 5-10mins after main lights out. Sorta like some do with far red for end of day treatment. I have a sneaking suspicion that this might work if youre having trouble controlling stretch with just adding it to the main white. Just like bk said there is genetics at play but ive seen with my own eyes that adding blue will help somewhat during stretch with shorter nodes. Also anecdote: my MIA growbuddy did a little experiment for me a bit ago. He had all plants in veg on 18/6 but interrupted main light for a bit in the middle, adding only 660 for about 15-30 mins. He got preflowers in veg even if veg times were maintained. By our normal reasoning this shouldnt happen. He was not happy about it and cursed me out but in the end they flowered just fine. From this i gather(or suspect): if you remove all light except for 1 type of morphogenicly active light the effects of that light on plant structure seems to be increased. The same thing happens if you over do far red end of day treatment, wild stretch and shade avoidance syndrome.
Again this is just theory but i think you could probably leave the plant with a "memory" of the blue light for the night cycle.
If you can control climate well you can also try negative diff: making sure that the temps of the first few hours of day cycle is lower than the last few hours of night cycle. I know this has been documented somewhere, try google cannabis negative diff stretch control and you should start hitting something. But as BK stated he tested this with no results so it it might be some genetic stuff involved aswell. Ill see if i can dig out something from the interwebs cause i know someone tried this recently and got somewhat of results, just not sure who or where.
Since im unattached to any grow now, i know bummer, if you try any of this i hope you hit me up and share your results, please. Experimental stuff like this is like crack for me, give me a nice sidebyside and its all im going to do for the next 2 months.