The default %JSON.Adaptor will meet most of your needs. One reason I found to modify it was just that I wanted to make its export to string and export to stream methods accessible via SQL. I have that on Open Exchange.

I'm not sure it'll be much better performance-wise, but you could make it something like:

AND ((TOURINFO.TNR = TNRHead.TNR) OR (TOURFIND.TNR IS NULL AND SGNRFIND.TNR = TNRHead.TNR = TNRHead.TNR))