Yeah, i guess it could be done in potions instead of signs, but signs are way easier! And that thread was talking about just making normal potions last 30 mins instead of 3-8 mins. And I agree that why pay that much when you can just make the potions!
This is OP "effects" for a price! Now i guess if you wanted we "could" select a few more OP perks and put them in the donor shops like:
VIP get LVL 1 haste/speed/jump for 60 mins
Prem - lvl 2 - 60 min
exlocusive lvl 3 - 60 mins
elite lvl 4 - 60 mins
supreme lvl 5 - 60 mins
ultimate lvl 6 - 60 mins
but that can always be "tweaked" later with more thought. I think however its done the base concept is easy enough and should not take much time at all to implement!