A Simpler Way
My formula for determining the "tipping point" was illustrative but impractical, since it was near impossible to learn the Average Investment of each Class A investor and sometimes difficult to even discover the Total # of Investors. Based on suggestions by @Daniel Han and @Arn Cenedella implying or stating that the magic number was just a 10% IRR alone, I went back to my whiteboard and worked through even more scenarios.
This time, I resolved to only use figures stated in nearly every OM distributed - the total amount of Raised Equity and the predicted Distributable Cash. I even used "real" numbers this time, from a couple of OMs and PPMs I have lying around.
In one example, the Raised Equity is roughly $9M and the Distributable Cash ranged from $600k to $770k and the final return with sale of $16M. Oh, and this deal limits Class A to only 15% of the total Raised Equity. Given that, we know that Class A requires ($9M * 15%) * 10% = $135k annually to hit their preferred return. That never changes.
In this scenario, Class B will earn roughly 0.5% less each year as quarterly distributions than a Traditional investor would have without the split. That's not a lot, but it's also not nothing.
But during the sale, Class B will earn 30% more than a Traditional investor would have with the same sale price, since literally 15% of all of the investors hit their 10% cap and then stop earning.
Doing all the math, we see that the actual tipping point is much easier to calculate. As suspected, it is simply a 10% annualized return!
To Sum Up
In practice, being a Class B investor would nearly always mean accepting a lower "income stream" in yearly distributions by some percentage dependent on how many Class A investors there are, since the times when operating income alone is enough to generate a 10%+ return while on-going is... rare. However, if you trust that the sponsor can deliver better than 10% overall (and in the market right now, even "underperforming" deals typically do), then Class B will tend to do even better than Traditional overall.
Are Class A / Class B Splits Wonderful?
It really depends on your risk tolerance. It gooses any return above 10%, but it will depress any return below that. And as @Brian Burke showed, in his typical insightful way, that the way the deal is structured could result in a TOTAL LOSS for Class B in a worst case scenario, while Class A made their full return!
I have two PPMs in front of me that illustrate both sides of this. One PPM says this in the case of any capital event:
That is, during any sale, all investors get their capital back first, with no preference between Class A or Class B. In a worst case scenario, where the sale is for less than the initial price, you are all losing money, but you're all at least losing money at the same rate with no preferential treatment.
The other PPM, on the other hand, says this during a capital event:
Oof. Yep, not only will Class A get their capital back first, but they also will first get their 10% return. So in a Black Swan event, Class A could even get 100% of their promised return on the investment while Class B lost everything!!
With that in mind, a Class A / Class B split can be as "safe" to your initial capital during worst cases as a Traditional route, but depending on how the PPM is written, it can be far riskier!
Is Class A Worth It?
That's obviously a personal decision, but I don't personally think so. 10% is a "debt fund" style return with a huge difference. If you actually hold the debt, then you have more opportunity to recoup your money during some kind of crash. If you are Class A equity, then your risk is the same as the rest of the equity stack and you do stand to lose it all. So you have the same risk factor as Class B but you are giving up any extra return to compensate!
Also, just like Class B, your money is locked in for the duration of the deal. In most "income focused" funds, your funds are semi-liquid where you can withdraw your capital after some lock-up period, typically of 1 year. Class A will have you locked up for 3-5 to maybe 7 or 10 years, depending.
The question, as Daniel asked, is "are there are alternative investments that will reliably yield 10% but without the drawbacks?"
In general, yes, potentially quite a few. Many debt funds will hit 10-12% and the big private REITs can also run in those numbers. I believe the Starwood REIT is returning 10% on average, for instance... and you can join in at any time; exit at any time after a year; and they go back to the 90s without losing any money (you also might not even need to be accredited).
But... yeah, I am finding it harder to find the "good" 10%+ income funds than the very many Class A offerings out there. The latter might be much much riskier, but boy is it made easy for you (as long as everything goes peachy-keen)