Someone mentioned pushing FP4+. I occasionally run it in Diafine at EI 200-250. One of the things I liked better about Plus-X was that it responded with more speed in Diafine, but it does still get a useful additional stop out or so out of FP4 (in practical terms, spare me the densitometery in this case.) Good for contrasty light, not so good for flat light regardless if it's dim enough to need the speed bump or not.
Better yet, just get a medium format camera. I'm at least half serious. Since I got my two medium format cameras I really only shoot 35mm in low light where I need a very fast lens AND fast film, and even that may decline once I get the 80mm 1/9 for my Mamiya. HP5+ will not show grain worth noticing in your 12x16 prints from a medium format negative, even a 645 one. Used equipment nowadays is pretty affordable.
I do use medium format as well, an RB67. I have two types of photography. My 'serious' mode when I'm out on my own. In this case I take the RB67 and tripod and can take all the time I want. The other, more common, scenario is that I'm out with the family and have to fit in what I can get. This usually means no tripod or the children start moaning about how long I'm taking, hence the 'no tripod, camera shake, use HP5' situation! This is when I take the 35mm kit. I've tended to gravitate to printing at 7"x5" onto a 10"x8" paper, for which HP5 is fine, it's only now that I've started printing bigger that the grain is starting to creep in, but then so is the camera shake as well! Perhaps I just need steadier hands and then all the issues will disappear.
The former dilution will have finer grain but less acutance, and vica versa. When having to trade off grain against acutance, I prefer more accutance. My current combo is HP5 with XTOL 1+2. My Normal ASA is 320 (by comprehensive testing).
Barry Thornton in his book "Edge of Darkness" spends a great many pages explaining and showing why achieving high acutance is more appealing than the sole pursuit of ultra fine/low grain.
The theory behind tabular grain technology sounds great, but if some of the grains (even a few) are not pointing in the right direction it destroys the integrity of the entire grain placement. I just don't like it.
The only 3 films from Ilford i like are: HP5+, Pan F+ and FP4+.
I prefer Tmax 400 over HP5 but i prefer Pan F or FP4 over TMAX 100, my favorite ASA 100 film is Acros 100.