Well, that's true on the surface, but if you want to have a little more fun with the conversation, its really not all that simple. I don't mean to sound like i'm calling you out BTW, just using this as an opportunity to chime in is all.
So really spool comes down to a lot of things, but we'll break them down and look at them individually, starting with the turbo anatomy. So really the single greatest thing affecting spool on the turbo is the turbine A/R, or basically the size of the turbine housing. A larger A/R will take longer to fill, since it is a larger volume, and will spool slower but will flow more at the top end as the engine starts to breathe. The alternative is a small A/R which fills quickly, and spools quickly, but hits a choke point as the engine starts flowing more air. Most stock turbo cars are fitted with very tiny A/Rs, because people don't want to feel any sense of lag, and this is why they almost always have no top end power.
So, how does that relate to twins Vs single? In a nutshell, as long as you don't hit the surge limit, you realistically could put a smaller A/R on a single turbo and have it spool quickly. I've seen ridiculous things on small engine VW and Hondas, like a .48 A/R 3582 lol. This is what a certain company that used to try to sell kits here did if you guys remember their dynos. The car was making significantly more torque than HP, which shows you the A/R is slightly undersized for the application. Sometimes that's what you want, but it just depends.
On the flip side, you could have a slightly larger A/R on a set of smaller twins, and just use the small characteristics of the turbo for spool and the larger A/R for better top end flow.
So now lets talk about the engine as it relates to singles and twins. A 3.7L motor is basically two banks of 1.85L. When you look at the turbos that spool relatively well on 1.8-2.0L motors, they're really topped out around 300-350 whp. Now, its not as simple as just saying twins on a 3.7 will top out around 600-700, but we can see that even with small turbos there really isn't a worry of flow, as even tiny turbos can still break the stock motor.
So now with the single, 3.7L is enough to spool a single 700-800 hp turbo with ease, so which is better? For me personally, i think less is more. Playing with VVT advance and timing ****** (i'll elaborate more on that if anyone cares) you can spool massive turbos very quickly, so i would prefer simplicity. A v6 is also naturally a "Flat plane crank," as the exhaust pulses are spaced evenly between the two banks, so if the headers are designed in as equal length as possible you have a very clean delivery of exhaust gas to the turbo. You don't have to worry about sizing too small of an A/R on a large turbo to get it to spool, and you wouldn't have to deal with the added complexity of twins just for the sake of response. At the end of the day, you would just hope whatever company you buy from used some science and did their homework
some of you will get that joke.
We could probably go more in depth with this discussion, but my personal opinion is that this motor is perfectly capable of spooling a single turbo that will make great power. Obviously the discussion of twins vs single will never be settled, because there is no right answer, that is just simply my opinion on the matter.