I was thinking of either getting a second 150W sun system (300W total) or getting a 250W kit.
I've never used HID light, but from what I've read you should have 50w/sq ft. For T5HO, CFL, low-quality LED: 40. For high-quality LED: 30. You have 38.
Before buying a new fixture you might try supplementing in flower with side lighting. You can get
high-quality LED "lightbulbs" from Home Depot for $7 each. (<<link showing how to mount) They have a 10-year warranty honored by HD. Just walk in and replace the bulb. You'll have to invest in some mounting/aiming/power mgmt hardware, but that's a permanent investment. Something in your bag of tools you can use whenever you need it.
If you put one 9.5w bulb in each corner in flower, that would raise you to 47w/sq ft. That would probably make a big difference. I like the lower 9.5w because I can add more, giving me more coverage for a non-scrog grow. 12w (75w equiv) might be better for you if you want to keep it at one bulb per corner. If you were going to consider 18w (100w equiv), I'd do two 9.5w bulbs in each corner for more coverage. But, that might be overkill for your 150w HID.
I'm sure it's just a matter of more light. I ran T5HO without sidelight. Buds got denser when I added CFL. Ran an 85w chinese (Blackstar) UFO and buds were airy. Added a 9.5w LED in each corner (pictured in the link above) and buds got denser.
What I like about this approach is that it uses commodity parts. You don't need proprietary fixtures, etc.
@mauricem00 uses
these less expensive lights (<<link). I haven't tried them yet. No 10-year warranty nor as easy as walking into your local HD. But, a good price.
If you added 4 bulbs, that would put you in a good position to replace your HID with an Area51 RW75. It's good for about 20x20". That would give you 37w/sq ft of high-quality LED. Or, go entirely with bulbs. 4 Cree 18w Par38 spots on top ($100 at Home Depot), two 9.5w on each side. That would be 36w/sq ft.