I don’t think SIP is doing a misleding marketing for Zirri exhaust.
Saying it is the best, depends for what you think it is the best.
The best is who gives the more power? Or the more torque? or the better power spread?
The best will be something that can do all this thing togheter but it doesn’t exists.
For example for a Polini 177 (for PX) it is very good for city and touring to use a Sito plus, it will pull easy, climb without changing gears. But on pure power is far behind the SIP, Taffspeed, RZ and so on. But none of them is better on low rpm. Or power spread.
The Zirri is the best for power at high rpm. For me is the best race exhaust.
Polini 130 is a very reliable kit, you could use also for touring.
For race you should do some barrel porting.
You should make the exhaust port a little higher. And the bases ports larger and then enlarge crankcases to suit. If you want to keep some reliability it is enough, it will help the kit rev higher.
If you want a killer machine then you have to work the barrel more. And the piston too. And many other things has to follow.
But I don’t know if you want to push it that far.
The Zirri short 4 is essential. If not they wouldn’t have made it.
Vespa has only 4 gears and with engines that don’t pull at low rpm you need a shorter 4. If not the engine will not reach the power target zone where it pulls better.
If you are building an engine to rev high then you need big carbs.
Standard Polini 130 with race crankshaft and ported cases with normal pipe not a radical as Zirri one, it works beautifully with 24.
In a racing engine 26 it is minimum. I used succsefully Mikuni TMX27 - 30, Keihin PWK28 and only for highly tuned PWK 33. And no filter of course. Even the best filter is restrictive and in a race engine we don’t want any restrictions.
For racing I prefer these giap. carbs cause are smother then dell’ortos. But they are big and need frame cutting. But if it is for racing it is not a problem.
But with bigger carb your lows will suffer more, so reed is in this case good. With a Polini 130 you won’t get more power from reed but better drivebility.
But I think that all this modification will kill the real nature of this beautiful kit.
I think to get the best from a kit we have to go its way.
Polini 130 was not made to rev high. It is a very very good compromise of power, torque ,high rpm but not radical in any of them.
It is not a radical kit. As Zirri are and Malossi.
But globally for me is the best one. It is the best balanced.
For me the best cooking for this kit is: race crankshaft, ported cases, Polini exhaust or some equivalent, and 24 - 26 dell’orto carb. This will give the best compromise.
Any other modification as the use of a more radical exhaust, bigger carb ecc, not followed by the right compensation either will make it slower or globally worse even though potentially more powerfull.
Can you immagine you make a modification in your engine that will make it gain 25%more power at 9000rpm, but less power at 8000 then standard. You will be very happy! But for too long gearing or other reason you will never get to that target rpm. Result : you will go slower. You will never see that power, it can’t be expressed in practice. The engine can’t get at that rpm. So I don’t think in practice you will be very happy to have more power.
You said you like reving engine but how far you want to go?
Tell me what you are ready to loose for power and high rpm, and I will be able to be less panoramic in my answer.