I can't say with certainty that you are incorrect, but I can suggest that maybe you are. I can say finding any consistent data on Miata cams is rather frustrating. It seems that everyone has different data, so it is very difficult to determine exactly what is going on.
OK, having said that, I'm basing this on the cam data listed in the NASA Spec Miata rules, posted here: http://www.nasaproracing.com/rules/spec_miata_rules.pdf
I'm also using the cam calculators here: http://www.wallaceracing.com/Calculators.htm
I know that Solo Miata list very different timing figures than NASA does, but the timing figures at Solo really don't match up to the duration numbers, so I chose to go with the NASA Spec numbers. I'll also say the NASA numbers seem to open the intake valve very early, but without any credible evidence to the contrary, I have to go with them. One more thing, it sure seems to me that Mazda chose to add about 4-7 degrees to the closing ramps, so I think that the factory cams are asymmetric; that is the raw duration numbers may be a little misleading compared to a symmetric profile. Now I'm not saying asymmetric lobes are a bad thing, actually just the opposite. But when I studied the data, I came to the conclusion max lift was not in the middle of the duration numbers, it's slightly advanced. If you subtract about 4-7 degrees from the closing numbers, the max lift numbers center up. Another way is to use duration at .050” lift to run the numbers, which is what I usually do.
OK, so looking at the data, using the stock 99 cam, we have the intake .010” open at 23.43 degrees BTDC. At just ½ degree after TDC, the intake is .060” open. The exhaust is closing, but is still open .070” at .85 degrees ATDC. That is a fair amount of overlap, my calculation say about 7.25 degrees at .050 lift. That is, about 7 degrees of crank rotation near TDC when both valves are open at least .050.
When you substitute another exhaust cam for the intake, it is even more. Still trying to verify this, but it looks to me that the factory cams have 100 degrees center line (intake fully open at 100 ATDC). Again, according to my calculations, if you time the exhaust cam at full inlet lift at 100 degrees, you wind up with about 10.86 degrees overlap at .050. That might not seem like much, but it's about 50% more. The original intake has timing of roughly 2/22 at .050” lift, the exhaust cam on the intake is about 5.5/25. The first number is degrees BTDC/the second ABDC.
The stock timing (according to NASA) is 23/51 56/43. These are .010 lift numbers, so you could maybe add a few degrees to them. I have found timing figures from Schrick, a European cam company. They list a cam with 22/62 62/22 timing. So maybe that intake opening is not so far off, but way different to 8 listed at Solo Miata. The Schrick cams work out to full inlet lift at 110, which seems a bit more realistic to me, but I really can't speak to that. I'm not saying any of these numbers are correct. I would really like to know if anyone has a different take or has measured their cam timing to see how they line up.