Fuji's published curves for Neopan 400 and 1600 look almost identical to me. There's certainly not 2 stops difference between them. Both films begin to toe at just under 10^-3 lux-seconds, by Fuji's own data, so I don't think that '1600' (cough, cough!) will give you any more noticeable shadow detail than 400.(posted 8342 days ago)
Ilford Delta 3200 and Kodak T-Max 3200 are both genuinely faster than 400 ISO, perhaps giving a real 1200 ISO speed.