Tiernumbers do not mean that tiers go up faster. Sometimes you can spend thousands of peds and not get a single tier-increase, and with other items with the same tiering-number, spend just a few ped and get one every click.
What they do mean, is the higher the number, the higher the chance the item tiers up with a click, compared to same item in the same situation with a lower number.
People also speculated the higher the number, the higher chance of enhancers breaking/disappearing.