A few thoughts about this.
First, many people think that a larger pipe will keep exhaust velocity up, but that is not how it works. If you were to compare two systems, one with a larger pipe diameter than the other but both close to the right size, the larger diameter pipe system would have lower exhaust gas velocity than the smaller pipe diameter system. There is a max size for exhaust pipe given your displacement and power levels. Anything above that slows exhaust gases down. Really trick systems will use a smaller diameter pipe right out the exhaust port and step up to larger diameter pipe 6 or 8 inches down the pipe. They are counting on the fact that the fuel / air mixture is still burning as the exhaust exits and as such it is still expanding. Because of that, the pipe diameter required for best exhaust flow is less right away by the exhaust port and increases further down the pipe as the air / fuel mixture finishes burning. But any sudden increase in pipe diameter / size will result in a slow down of the exhaust gases. Sadly, the stock exhaust crossover is just such a change in diameter / cross section. But the stock system is built to be cheap to manufacturer while working well enough. After market systems should be able to work better. But how much better can vary.
Next, exhaust gases do no flow out the exhaust in a steady flow, they are pulses. Between the pulses there are waves of negative pressure that flow back up the pipe. Since the Guzzi V twin doesn't fire the two cylinders at the same time the pulses of exhaust from each cylinder are never trying to flow out the pipe at the same time. If the system works well, the negative pressure waves flowing back up the pipe from one cylinder will help the other cylinders positive pressure exhaust pulse exit quicker, which then can cause a stronger negative pressure wave to arrive at the exhaust port of that cylinder. The interaction between positive exhaust pulses and the negative pressure wave happens both between the two cylinders at the crossover and at the exhaust valve. But because a 90 degree V twin doesn't have an even firing interval between the two cylinders the affects of exhaust negative pressure waves are more likely to be back at the exhaust valve from that same cylinders exhaust pulse. But there are systems with complex crossovers, like the Ducati "spaghetti" system that have two separate crossovers to address the differences in timing between the two cylinders.
All I can really say is back in the day I wanted a two into two system for our Ducati 750 twin racebikes (two separate exhaust pipes, basically). I did not think that a two into one system, or a two into one into two system, would work best on a 90 degree V twin. But we ended up having a system built by a guy who built exhaust art. And he didn't build what you told him, he built what he knew would work. We ended up with a two into one system. It maybe did not make the most peak power, that probably would have been from a two into two system, but it made more power under the curve and as such was ultimately faster than other bikes that claimed to make more peak power.
Another really cool exhaust technology is merge collectors. If you want to learn more, google that. Merge collectors put a lot of effort into merging the different exhaust pipes into a single pipe while controlling and maintaining the size / cross section the whole way. They don't really have a lot of benefit for a 90 degree V twin like a Guzzi, but they are exhaust art.