Are there any downsides to opting for portfolio margin vs Reg T margin in a long-only stock account? No hedging or derivatives. I am currently using Reg T margin and limit overnight leverage to 1.5x. If I opt for Portfolio Margin, the current margin cushion is much larger, but is there a chance it could ever be adjusted to less than Reg T margin during extreme market volatility?
Depends on how your broker calculates PM. I think IB does not disclose their formula. However, there is a tool which shows you the margin requirements with both. I think you should get it. Unless you have a portfolio of only short vix calls, pm should give you more margin. The real benefit of PM is the ability to be short more options.