RE: LETF
With SSO scaled to 1x exposure i.e. half in SSO, half in bonds, those weightings will quickly deviate. If prices rise the you move to 51/49, 52/48 ... etc and vice versa if prices decline. That amplifies the upside, attenuates the downside. Hence if stocks drop 33% a 3x wont have lost 100% but something less.
You can deduce the effective borrowing rate by setting a period, say a year, and then calculate half the 2x change over that period and compare to 100% 1x change. Twice the difference is the amount that the 50% bonds allocation would have to make to close the gap. For one year long periods calculated for each and every day you'll see that that varies quite widely (could be +20% or more at times, -20% at other times, and more usually is high when volatility is high), but broadly averaged around 2% since 2008. For shorter periods, calendar months and the deviation is less and the amount was smaller (0.277%) which pro-rata to 12 months also = 2.8%. i.e. broadly since 2008 if your bonds averaged more than say 3% then overall LETF/Bonds produced a higher reward than SPY. Another way to potentially bolster rewards is through choice of rebalance timing.
Fundamentally you want to rebalance on a dynamic basis rather than at set intervals and the closer you can get the rebalance timing close to peaks and trough so much the better. AIM tends to do a relatively OK job of such indications.
For bonds you want some liquid, ready to be deployed if you need to buy more SSO, some can be less liquid and possibly earn a higher return. Diversify bonds and you can pick whichever is the more appropriate to sell as and when additional SSO shares need to be bought. Beating the index in effect distils down to getting the bond half earning a reasonable return. IME in the current low interest rate environment bonds earning a couple of percent more than needed is relatively easy, which value adds around 1% - which broadly covers costs and leaves a little surplus i.e. gross index total returns plus a little after costs.
Timing of rebalance trades has the potential to add more. But that's no different to sometimes using gearing/leverage, sometimes deleveraging, drifting from say the equivalent of 120% exposure at times down to 80% at other times and broadly averaging 100%