For backtesting purposes you might create your own historic 3x stock yearly returns figures on the basis of borrowing at the start of each year at T-Bills + 2% (or whatever) assumed cost of borrowing. Have $100, borrow another $200 when T-Bill yields are 4%, so assume 6% cost of borrowing, deduct that ($12 interest) from the combined $100 you have and $200 borrowed - $12 to leave $288 invested in stock at year start. At year end calculate how much that £288 stock value had risen/fallen to and repay (deduct) the $200 borrowed amount for your overall total 3x gain/loss that year. Rinse/repeat. That yields dissimilar outcome to regular 3x funds that rebalance daily but broadly that washes. In some years that may result in a negative outcome, 1931 and 1938 IIRC so needs to be combined with other assets that you 'rebalance' with to cover that. i.e. perhaps a asset allocation such as 25/75 3x stock/gold.
One way to assess that is to assume 50/50 1x stock/gold as your borrowing offset, where broadly that stock/gold combination might offset the cost of borrowing.
25 in 3x stock = 75 long stock exposure, 50 borrowed. Alongside 75 in gold.
Take 50 in combined stock/gold to offset/negate/pair with that 50 borrowed
Leaving remainder 50 long stock, 50 gold [additionally 50 borrowed and 25/25 stock/gold exposure to negate that]. Perhaps benchmarked (in a broader/multi-year manner) to a 50/50 stock/gold
https://www.portfoliovisualizer.com/bac ... NHxNHry0MC
Since the US opted to sanction Russia much faith/trust has been lost and as part of that bitcoin has transitioned to being priced to somewhat reflect a 3x tech stock type holding. Since 2021 those with high wealth might prefer to hold 25/75 bitcoin/physical gold where the bitcoin is offline (outside of the regular system). Of course rather than just bitcoin alone other/multiples may be held instead (Monero/whatever), where a five way split dilutes the individual 'crypto' asset risk down to 5% each. As part of that crypto is inclined to be bought but never sold (not moved back into regulated/controlled exchanges). Sooner or later a 3x stock like asset (bitcoin for instance) will have a very bad year, perhaps lose 90% of its value, in effect a large chunk of the value is transitioned over to other assets (such as gold) 25/75 bitcoin/gold where gold rises 30% in reflection of stocks being down -30% and 3x stock (bitcoin) being down -90% -> 2.5/97.5 bitcoin/gold holdings. No nominal portfolio value hit but with much of bitcoin weighting having been redued, transitioned over to gold. To liquidate bitcoin into cash some of bitcoin is P2P swapped for crypto gold (XAUt perhaps] and physical gold of the same amount/value is sold for cash in hand.
Fundamentally you're just holding a 1.5x leveraged 50/50 stock/gold like blend. In contrast borrowing to buy more Treasury bond exposure is somewhat borrowing from yourself along with setting light to some cash.
