Marco, when you say that you "visually" checked the densities, are you saying that you did measurements of the tonal ranges, not just how dense the neg looked, with a densitometer? Checked the total range, including FB&F, not just zone I? Higher speed films tend to look denser than slower speed films regardless of reciprocity. I am willing to bet that the FB&F of the T-Max400 is already higher than that of the Acros, and would lend a look of greater density to begin with.
In looking at the reciprocity failure data reported by Kodak and Fuji, for exposures of 100 seconds in length, Kodak does not indicate ANY times longer, the correction for Tmax 400 is 1 1/2 stops. So the effective EI drops to about 150 for a less than 2 minute exposure.
For Fuji Acros the data given is for exposures up to 1000 seconds, or nearly 17 minutes, and the correction recommended by Fuji is only 1/2 stop, or an effective EI of 50. However the data also indicates that NO compensation is required for exposures shorter than 2 minutes. Whereas the T-Max 400 a +1/3 at 1 sec, a +1/2 at 10 seconds, and a plus 1 1/2 at 1000 seconds. This indicates to me that compared to the fuji, there is a more rapid decrease in film sensitivity as exposure time increases, whereas the fuji seems to stabilize. One could argue that on exposures of hours length, that the trend of increased reciprocity failure of the T-Max 400 would increase and could catch up to the Fuji Acros in actual sensitivity. If that were the case or even if there was still a slight edge in speed in favor of the T-Max 400, one still has to consider is the image quality of T-max400 worth the slight increase in effective speed?
Fuji Acros has a RMS granularity value of 7, and a resolution of 60 LP/mm, T-max400 has an RMS of 10 and a resolution of 50 LP/mm. Personally, I'd rather wait a little longer, or open up a half a stop, to use a superior film.