What can Wealth do?
Ok, let's quantify it a bit. What would massive wealth do for a character? First of all he'd have all the goodies he wants. He'd run around in the best magical gear available, even more than his level repective wealth should allow.
He'll hire henchmen to help out his leveling by supporting him in combat and bringing him to the dngeons faster (travel time reduction by horse or magic teleportation), he will hire Librarians to check for the solution for puzzles and locations of dungeons, he can hire the best teachers to give him the spells he wants... and raiding dungeons will keep a steady influx in money for him.
Let's face it: the biggest time consuming factor for a hero group is not the dungeons itself, it is overland travel. They spend weeks and months to get to the next town and then days to investigate till they find the dungeon.
Our wizard pays some cash and BAMF he is in the target city or close to it, then pays some cash and has the info he wants and some more for a few hands to carry the loot, then he enters the dungeon.
What took the heroes maybe 6 weeks, he did in 6 hours. The dungeon itself takes them the same time, but not spending 6 weeks of travel and research between dungeons and assuming doing a dungeon takes about half a week, this means the wizard does 13 dungeons in the same time the heroes need to find and clear one.
He only needs to accumulate 85,000 more XP to be lvl 20 when the heroes are lvl 15. That is 181% of the XP the heroes accumulate. So he just needds to do two comparable adventure for each adventure the heroes do. Which... can be done pretty much by eliminating or greatly reducing travel time and outsourcing the research.
How much overkill will his wealth become?
As he started with 3 million gp when standard wealth at lvl 1 is about 150 gp and assuming a simmilar factor of 20,000 times the standard wealth, he'd end with LOTS of cash: assuming PC progression he can buy at lvl 20 a country (15,200,000,000 gp wealth) and still a city if you use NPC progression (4,400,000,000 gp of wealth). With that funds he could levy an army if he was in the mood for it, build a castle and still run around like a christmas tree of the best magical gear available.
Now, here comes the problem: He has all the moneys he might need and want, and is 5 levels atop the group. With his toys he is clad in the best armor available for a wizard (which is probalbly a very espensive ring of magical armor and ioun stones), and he has all the deadly lvl 9 spells... To put it in comparison: He became a dragon in human shape. He might have prepared and a bound large demon or two, that each alone might be troublesome for the heroes and he has most likely henchmen, informants and followers of similar level to the heroes available should he need them.
And that does not count his right-hand-man. Not his assasin or most trusted goon, but the most fearsome man a richman can field: his accountant. The guy he hired to control all the wealth he doesn't carry around in magic toys: The wizard literally has the funds to own a city. That means everybody in the city owes him or his quartermaster because he gives them the ability to live a live at all (shelter, protection, allowance to run buisiness etc) which explains the huge ammount of wealth (tax). Sure, they might not like him (though a smart evil overlord owning a city or metropolis or country will make sure they do), but rebelling against the one person who's funds keep the city/country afloat at all is suicide. What happens if the wizard disappears? The money does too, the prosperity of the city is gone in moments, anarchy and downfall. So just by the virtue of being the souvreigh and the heroes being enemies of the souvreign, the heroes become personal enemies of the population if they threaten the Status Quo.
Wealth is the best superpower. Because money=friends, backup, information and all the toys.