So, here's my theory why LEDs sometimes have a cone of light beneath them and may be considered dim:

1. Some are dim-Some LEDs are dim however standard LED streetlights can be much brighter if installed differently.
2. Their fixtures don't reflect much light-With HID fixtures, the inners are usually reflective letting the lights bounce around. LED streetlights are usually flat panels and the
   diodes just face down so there's a nice little cone of light.
3. LEDs are never droplense, the gem400a2's I've seen with droplenses are very bright and would be brighter if they had brighter LEDs however that also means light pollution issues
   however in the city there's light everywhere anyways.

The smartest way should be to just not change them out then the second smartest would be to get some quality paddle leds and smack em in HID fixtures as replacements.
If my theory is correct then I think it should work.

LED lanterns do actually have optical beads mounted just below the diodes, but what people mistake as a ‘dim’ lantern is there being no light spill on the surrounding area.
Our town was SOX for near on 37 years, and we got use to seeing the road and surrounding areas lit with light, the first night we went all LED, (December 2016), I too thought they were dim, but within the first week I started to realise the light was just on the road, and the houses were in darkness due to the lack of light spill, and it’s this lack of light spill that gives everyone the impression of dimmer street lighting!
However the actual light that’s put on the road surface is still the same as what the SOX/HPS put there also.

Such lanterns would be jokes and not proper streetlamp.
But my guess is, it is the use of wrong specs lanterns, too short poles or improper installation.
Normally the optic is designed to provide as uniform illumination on the target surface as possible with the given light source and reasonable size and cost. With LEDs this leads to near perfect uniform rectangle underneath the lantern. But the problem often is, all these designs rely on the way how the lanterns are installed, mainly the aspect ratio between the pole distance and height: Standard says the pole distance should be 4x their height. If they are longer distance from each other (or someone used shorter poles than their positioning was designed for), the space inbetween is not adequately illuminated. And because the LED optic uses to be quite precise, it leads to a rather sharp cutout on that edge, so when the distance is wrong, it leaves not illuminated patches inbetween.
Same problem would be with many modern high efficiency HID systems as well, as with the modern precise faceted optics the nearly ideal beam pattern was also feasible.

Many LED streetlights I've seen used to have a lack of light spill, the LED streetlights used in my area are very bright when installed correctly (mostly atb2's). On the big road near me they use way to short poles on a road that's 5-7 lanes (including median). The light from most streetlights don't usually reach the middle of the road if I remember correctly from the shorter days. There is a section where there are two atb2's across from each other on the road and it is a very bright section of road and I've only seen one atb2 ever turn purple.