This is going to sound like a commercial for Octane, but I have no affliation with them other than being a customer :-)

I use Octane (made by OTOY) and I really, really LOVE it.  As someone mentioned above it does do SSS, but not displacement (yet, although the renderer is continually being improved and having features added). 

I've used Poser for years and years and the hardest part for me has always been the lighting.  My favorite part of Octane is how easy the lighting is, and how easy it is to make great renders with just very basic lighting setups.  A few months ago I got very frustrated with Poser, or maybe more fairly, the process I had to go through to get the lighting right in a scene, so I looked at some alternatives.  I found Octane and even though it was a little bit pricey I thought I would take a chance.  I am really glad I did because it's reinvigorated my enthusiasm for Poser and I've been having a ton of fun with it ever since. 

Octane is CUDA based so render times depend on the graphics card(s).  Because it's CUDA based, there are also certain limits on the numbers of textures that can be loaded, and the renderer uses the video card RAM, not the system RAM, so that's important too.  So for instance, I'm using an Nvidia Titan card so I can load up to 6GB of memory and 144 rgb textures and 68 greyscale textures.  (If the same texture is used more than once it only counts as one instance against the count).  This has been way more than enough to pack a lot of stuff into my scenes.

My experience has been that it is blazingly fast compared to firefly.  Depending on the lighting setup, I can get high quality renders in as little as 2 to 3 minutes.  Seriously.  I think the longest render I ever did was an hour and a half, and that was due to very complex lighting I set up (and I'm sure my own ignorance in how to optimize the scene).  I'd say my average render time for very high quality renders (much higher than I could do with the native poser render) is probably about 4 minutes.

Anyway, that's my 2 cents on an alternate to Firefly :-)