It seems like their regular margin is 25%, but for shorting it's 30%, and sometimes 40% or higher. Has anyone seen them be flexible on this, if there's a track record of not being reckless?
It's much higher than 40% for many equities. 80% and 100% is very common these days. Doubt you will get any improvement with them, their margin requirements have been getting stricter every year and effectively you can soon expect only 2:1 or less intraday.
Consider a hedged position which I have found to lower margin requirements. It may not fit your style of trading but works for me to obtain margin relief.
If the underlying has options, would a put debit spread fulfill your needs? Whenever I want to short something and hold for more than a day, I first look to selling calls or buying a put spread if the underlying is volatile.
They can't offer more leverage than allowed by FINRA, only less. To do better you need to qualify for a Portfolio Margin account. That maxes out for equities at 15%.
If you are searching for low margins, then Interactive Brokers are not the best brokerage for that. Interactive Brokers have low fees and a lot of instruments but low margins are better found elsewhere at places such as Saxo Bank and Exante.
Low margins or margin rates? Margin rates - the ones you mention are higher. Margin leverage - for US stocks as mentioned by Robert FINRA sets the overnight rules and maybe you'll find some firms offering more leverage but I suspect not if you have over 100K and qualify for a portfolio margin account where you are not subject to RegT and get the benefit of lower margins the more diverse your portfolio is.