4. Analysis:
The results between the two you can see are remarkably similar. If you got 8% in the market, it's even tighter. If taxes go up from that 20%, it favors BRS since more of your money isn't taxed.
The analysis is flawed in that you're comparing apples to oranges. There is nothing magical about BRS that enables a person to contribute to TSP. There's nothing magical about BRS that allows a servicemember to transfer his TSP account post-service to another 401k. The only thing that BRS changes is that it reduces your percentage of retirement pay (and amount your spouse will get if you pass away or are killed on active duty as a result) per year of service and in turn provides a match for money invested into a retirement account, along with a continuation bonus because the military won't match your first few years of service anyway.
If you're going to compare apples to apples, you need to assume that the person invests the same 5% into TSP regardless of whether he is using high 3 vs. BRS. And if he's doing high 3, that money should be going into a Roth TSP.
So High 3 is 50% of base pay at O-4/O-5 + 5% invested into Roth TSP * compounded growth - income taxes on pension
BRS is 40% of base pay at O4/O-5 + (5% invested into Traditional TSP + 5% government match ) * compounded growth + continuation pay * compounded growth - income taxes on pension - income taxes on TSP
At that point, you're really just comparing if the 5% match + continuation pay minus taxes post-retirement on traditional TSP will outpace the extra 10% guaranteed in retirement at 20 years, tax break from the Roth TSP, and 2.5% for each year beyond (pro tip: it doesn't, and that's the whole point of the change).
Additionally, a post-retirement O-4/O-5 is going to be making in the realm of $40-50k in retirement in present year dollars. For a married person with no (or grown) children, that puts their federal marginal tax rate much closer to 12% than 20% (16% if single). This doesn't say anything about state taxes, which is something you may not be protected from after you retire from the service. And in some states, like NY, your federal pension is not taxed for state income tax, but your income from a 401k retirement account is.
Now there's more to this equation than just money, like whether you want to take the 5% match now because you may leave before 20 years regardless of whether or not you want to, even though it will be less money in retirement. Or whether you just want to say f-it and spend the extra 5% pay on hookers and blow knowing that you have more in retirement from a military pension anyway. Or whether you'd prefer to diversify that investment into something other than stocks that you can't touch until age 60.