I just did this in WPX8 (I don't have anything prior to WPX6 on my computer):
1. At the top of an existing document I set new paragraph spacing with Format>Paragraph>Format>Spacing between paragraphs = 1.5 lines. (I first made sure the paragraphs were separated by a single hard return (HRt).)
2. I use my mouse to select a single paragraph (to include the hard return code at the end) and applied a single paragraph border with Format>Paragraph>Border/Fill.
The spacing between border lines above and below the paragraph is as expected: There is no additional vertical spacing between the lines and the paragraph text.
Perhaps there is something in Reveal Codes that is causing an uneven vertical spacing?
Perhaps you could test things by opening a new (blank) document and typing (NOT pasting) a couple of short paragraphs of plain text into it. Then use the 2 steps above. Does this work -- or do you see the same problem?
I cannot replicate it.