Does a turntable make a DRAMATIC difference?

Yes. 

Not saying you have to spend $100K or near on a turntable, but once you've heard an exceptional turntable setup on a suitable system it's apparent that there is much more to a fine table than just spinning at the proper speed. Analog playback is a system of accurately measuring the signal recorded in the groove and creating an electrical signal which accurately recreates it. There's a lot of technology involved in the process which has been refined over many decades. Just being more expensive doesn't necessarily equate with better performance but truly high end performance and being able to achieve it doesn't come cheap. Of course you can still enjoy a lesser system but once an elite performer is heard it's hard to go back.

I’m not super educated on newer equipment, I’m older and therefore prefer vintage equipment, over the years I have been fortunate enough to keeping trading up and up and up to where I am now. I’m running 1963 “The Fisher” 800 C AMP, Turntable: DENON DP 62L with a Audio-Technica VM760SLC cart , speakers are Cerwin Vega  D-3 . As stated I’ve traded up from low end pioneer receivers, all kinda of turntables, and speakers and I can say I have heard a incredible difference, with my current set up I’ve heard things I’ve never heard before, so I think the answer is yes you can get a dramatic difference listening experience in upgraded equipment.  

Definitely upgrading the turntable can make a substantial difference but right now, I would be more concerned with all the mods done to the Rega.  All these after market mods may be simply mucking up the works and not an improvement.

Unless you put huge amount of money into your tt set up it'll never sound cd quality.