Ok, first of all, we don't use sub-scatter maps, so all the details have to come from diffuse, normal and specular map.
Your normal map seems very flat and the base model seems very detailed, I guess you should move many more details into the normal map. The stem should really only be a single continuous area on the texture maps, just like the head. You shouldn't be able to look through the stem, make sure the vines are really just bumps on a continuous surface. (When I said the outer layer of vines could remain in the low poly version, I meant that the stem would not be flattened to a dimple cylinder, but it should still be significantly closer to a cylinder than to the high poly model.) The same applies to the head: The convex shape of it allows you to put nearly all of the detail in the normal map and leave only very rough bumps on the model. 8.5k isn't far from what is acceptable but still I guess you can easily get down to something in the range of 2-4k without sacrificing much ingame quality. The stand should also be more prominent on the normal map.
Speaking of ingame, there's no need to paint the gills black as the lighting model will do this in most situations except when the leech is built on top of a light source (in which case the gills should be the only thing illuminated). Just give the leech a dull diffuse map and a strong normal map and the game will do the rest.
Have a look at this and this post for an example of how details should be spread across mesh and maps.