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The thought experiment involves twins who are identical except for SHBG. First off, the rate of decline in serum testosterone after an injection of a testosterone ester is independent of SHBG. Instead it is mainly a function of the ester, the carrier oil, the additives, the injection site and activity level. So basically, if the half-life of testosterone cypionate is five days then the twins have identical amounts of free testosterone and see their levels halved after five days—assuming immediate post-injection peaks. What's interesting is that while total testosterone for each twin drops by roughly half in the five days, their levels are different, and the twin with higher SHBG has proportionally higher total testosterone throughout the injection cycle.

It's a myth that lower SHBG leads to faster metabolism/excretion of testosterone. Consider another thought experiment: Each twin has a constant slow infusion of testosterone such that there is no variation in serum levels at steady state and each twin is getting 5 mg of testosterone a day. How much does each metabolize and excrete? It has be be 5 mg per day regardless of SHBG, because at steady state they can't be using more or less than they're receiving.