Other than size/space, do higher tech planet systems produce more?
or is say does a research value of 15 produce differently if its produced with higher tech or is 15 just 15?

as you research building tech the structures you can build will produce more and generally larger but it will take less of them to reap more benefits

As you increase the tech tree for your planets the new structures typically get very bit, but they have a better resource to size ratio than the lower level ones do.

Actually answering your question, a production of 15 will always produce 15 research on an average resource planet (regardless of any building combination used to obtain 15). Higher resource planets grant a percent modifier bonus to production. (view the wiki 'planets' page for more details) If it has been 5 days since you acquired the planet the production doubles. If you're a vygoid, the research production is magnified by 1.2 times.