You are right, the percentage gap will fall away as the price of PPT increases in value. With the lower prices now compared to what they may be will still give you an advantage as opposed to those who just invest in fiat. Buy PPT at low prices then compound your Pokens.
PPT will always give you the edge over fiat but as time goes by the edge i think will be due to buy back/burn each year. So your allowable investment will increase by (say) 10% each year as compared to fiat investor who do not benefit from the yearly increase.
Investor who hold PPT and not use the platform will hold PPT for yearly price appreciation through buy back/burn. If you want to trade PPT then I think it will be difficult to make money because by then no-one will want to sell when they realise the money they are making on the platform.
So it is a good time to buy PPT when the price is low compared to where the price may be after the beta. Fingers crossed that beta will be a hit. Would want to invest in fiat when as a PPT holder you get just that bit more even if you buy PPT after the pre-ICO and the price has gone way up.