The stock Vans cowl is a pretty "rough" product with a lot of trimming, sanding and fitting to get it right. Pretty much ANY fiberglass product from Vans could be done butter (cowling, canopy top, doors, fairings) so when possible, many builders choose to go with after market products. If you tell Vans you don't want an item, they will leave it out and discount the kit price accordingly. If you plan ahead, its a great money saver. If you discover later on, you can always send back the unused part for a refund (minus 10% restocking usually).
As a trivia matter, the fiberglass products sold by Vans are outsourced to a local company in Oregon. That company primarily makes fiberglass shower enclosures for the home market.
As for the Show Planes benefits, that remains to be seen. But I suspect the quality of the fiberglass and fit will be greatly improved. However, they alter the air intake, induction and exhaust exit so the long term affects is unknown at this point.