yes, be real careful using zener diodes. I broken a piston in a new rebuild motor because of it. The main problem is the tolerance of the ZD. When I do it now, I buy like 20-30 of them and load check the break down voltage.
With an external adjustable pressure source, measure the MAP voltage output vs. pressure BEFORE and AFTER wiring in the ZD. Note what the output voltage is before adding ZD at about 14 PSI. Then try to match a ZD to that voltage output at the BOOST you want to run. You will get an approxiamte idea of the changes from the voltage plots. The ZD will start to lean out the engine even at lower boosts. Start high and work to a lower voltage.
15 PSI boost may still be too much on a hot day, dependng on your other mods, also allow for boost spikes.
You really should have a good wideband like INNOVATE and AUX box to get a real time log of O2 vs boost. It's easy to miss the boost spikes. I had a cheap narrow band, that didn't give me any lean indications, it was false security.
Going to 3 bar MAP and matching computer etc. is probably the best way to go. Cheaper than an engine rebuild.
You shouldn't even have to use a ZD if you are not running more than 14-15 psi boost.(Maybe to take care of boost spikes). A good G-valve set-up would be good 1st addition. You will get better torque at lower rpm with it. Leave the map alone for safety.
Good luck, be REAL careful!