Sounds good in theory,
Offcourse you would have to balance the benefits of the potential extra returns, against the risk of the potential increase of loss.
eg, if there was a big down turn in the market that caused you to become a forced seller at a lower level, you might turn a temporary 25% paper loss, into a 90%.
if your portfolio was 30% your equity and 70% margin loan, and it dropped 25% and was margin called, it would wipe out almost all of your equity and you wouldn't have the portfolio any more to participate in the upside.
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lesson is, don't push your margin loan to its limit LVR's