Does upgrading your iMac's traditional HDD to SSD improve the graphics performance?
It depends by what you define as "increased performance."

An SSD will increase performance with respect to getting digital assets off the drive and into memory and the CPU.  If you were having lag due to slow read/write times, an SSD will help.

If you have framerate issues, an SSD will do nothing to help your situation.  If your GPU chokes on 30 fps, it will still choke on 30 fps after an SSD upgrade.


  I have already gone through this post (Would upgrading to SSD solve my
  performance issues?), but this guy has a MacBook so that doesn't
  answers my question!


Technically speaking, that iMac is closer to a MacBook Pro than an actual desktop because it's using the Intel® Core™ i5-4260U Processor which is a processor aimed at the mobile (laptop) segment.

The HD5000 video chipset is shared video memory so even with 8GB RAM, you actually only have 6.5GB max available to you.

There's no amount of RAIDing or setting up external GPUs that's going to overcome this performance deficit.  This is like trying to supercharge a moped with the expectation it will haul freight.

Your best option is to get the most powerful older Mac Pro with a proper GPU that you can afford rather than try and bodge a new iMac into something it really can't be.