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That's not really true. Any changes you make to the turbo can significantly alter its efficiency. The other aspects of the vehicle's intake and the environment also alter how much extra heat can be dealt with. The figure 14 to 16psi was based on how hot the out coming charge is and the stock intercooler's ability to cope with it before the diminishing return become greater than the risk of damage. The 57 trim is much more efficient and capable of moving a much greater volume of air at higher pressure with less heat. With the compression drop you see from a 2mm HG and a good aftermarket intercooler you can get away with the turbo pumping out much hotter air than normal and still see a reasonable increase in power. It's not all about pressure though, there are a lot of people who push 18 on the stock CT-26, drop i an upgraded CT-26 and are amazed that at only 10-12 PSI they're making the same amount of power.

My concern is the pistons. If they're stock they may be a relatively soft aluminum allow (not even hypereutectic) which makes them a lot more susceptible to detonation damages. Running rich doesn't mean much.... Many people "think" they're running rich but are often in fact running too lean when it counts. Do you have a wideband? What's your AFR range when boosting? 17 to 18psi shouldn't be a problem unless you're driving in killer heat, running lean, using cheap fuel, have excess carbon buildup and so on. I wouldn't try to go much beyond that though... I could get away with more given my altitude.