how much difference does differene does it make for a 19lbs wheel vs. 14 lbs wheel on the overall performance on the track if one is running R compound tires?

It makes a huge difference. Reducing rotating mass is arguably the best single change you can make to improve handling.
My suspension feels completely different when I bolt on my track wheels. Having said that, I have two new sets of custom, light-weight 17 x 8 Kodiak wheels for sale.

Here's a whole page of articles/forum threads about unsprung weight. Some of it is pretty good...

http://forum.miata.net/vb/showthread.php?t=40082

Trust me, the stock wheel/tire setups on Minis are veritable boat anchors, especially if you get beyond 16's. Peruse those articles to understand the various reasons that this is NOT a good thing.

Do it! Lightweight wheels make a huge difference in handling, acceleration and braking. I used to have a set of Kosei K1s on my E46 BMW just for autoross/track days.

You should feel a big difference in the car, on the track or off. I installed OZ's on my Justa, and even though that car sees no track days, I felt a large difference in how the car handled. It's just as noticeable as the difference between the OEM runflats and upgrading to a good performance tire. I will let you know my track experience after I run my new light Enkei's along with my R-comps April 12th @ Lime Rock.

I have to totally agree with the comments above. I just had a set of O.Z. Ultralagggeras put on my car in place of the WEB Spokes and all I can say is WOW!!! The handling is totally different not to mention they look fab!