I don't agree with this line of reasoning, as it is material who does the borrowing. Only the entity that bears the borrowing cost (aka the carrying cost) of the investment can deduct that expense on their taxes.
So if the mutual fund "does the borrowing" , they get the tax deduction, not you, the mutual fund investor.
So, while your example assumes that both scenarios pay tax on an $8 profit, you're negating the impact of $2 interest expense, which is deductible by the investor if he borrows / leverages, but not if the mutual fund does.
Also, consider that your example falls down if there is a loss in a given year. In both instances, the loss can be carried forward, or carried back to offset gains from previous years, but the interest cost reduces the investor's tax burden regardless of the performance of the underlying investment.
Finally, consider that for high-income earners--say 1%ers (i.e. earning $293K+ in Canada)--who have typically already exhausted their RRSPs and TFSAs, and the associated tax benefits of those registered accounts, leveraging in their non-reg account creates a much needed tax deduction, while increasing their overall asset base.
Now, I'm definitely not saying that leveraging is for everyone, but IMO it has considerable benefit if you skew towards the top end of the income game and are fine with holding / using leverage through the market drops.
Wills.
I am not sure you understand how it works. For simplicity, assume stock (call it "market index") costs $100, generates 5% return, and borrowing costs is 2%, you have $100 of your own money to invest and there are two mutual funds. Fund 1 invests all money into that "stock". Fund 2 for each $1 it gets from investors borrow $1 and invest these $2 into that stock, You have two possible leveraged investment strategies:
Strategy 1: Borrow $100, invest $200 into fund #1, generate $200*5%=$10 return, pay $100*2% interest, have $8 pre-tax profit and $8 taxable income
Strategy 2: Invest $100 into fund #2, fund #3 borrow $100 an invest into the stock, the stock generates $200*5%=$10 return to the fund, the fund pays $2 interest and distribute the remaining $8 profit to you. You get pay $8 pre-tax profit and $8 taxable income.
Both strategies result in the same after-tax profit of 8*(1-tax rate)
Now, if you pay 3% interest while the funds can borrow at 2%, the first option will lead to $10 return, $100*3% interest, $7 pre-tax profit, $7 taxable income, and 7*(1-tax rate) after-tax income.
Assuming the stock return is a random variable, the above is for expected values (in case of a loss, both you and the mutual fund can carry losses forward the same way) and the beta of your investment is also the same (=2) in both scenarios (in fact, not just beta but the realized return in any possible scenarios)
So, the only difference is the borrowing costs