I have been looking at the entire tonal range, and what I saw was consistently slightly more shadow detail for TMax 400 in all negatives up to 8 hours, and less contrast than in Acros 100. This last point may be effect of the development as per above.
Please do note though that for all real exposure times I tested (7.5, 15 and 30 minutes, 2, 4 and 8 hours) TMax 400 and Acros 100 were very close, although Acros 100 was maybe more close to TXP 320 for exposure up to 2 hours. In fact, if I had seen the negatives, and didn't know what they were, I might have assumed they both had the same ISO speed, just received a different development...
The thing I am disputing is the general "myth" that for very long exposures times, 100 ISO films will be "faster" than 400 ISO films and you will thus have shorter exposure times with 100 ISO films, regardless of what 400 ISO film you are shooting.
While the "myth" may be true for a film like Ilford HP5, some films, even in the 400 ISO range, just have far better reciprocity failure characteristics than others, and I am especially referring here to TMax 400. As a consequence, the break even point between 100 and 400 ISO speed film may not be reached within any realistic exposure time scenario. This is the case for TMax 400 against any 100 ISO film like TMax 100, 125 PX, Fomapan 100 or so. TMax 400 will beat these films at exposure times up to 8 hours... Again, Acros 100 is the big exception, as it almost equals TMax 400.
To illustrate this also, I included Ralph Lambrecht's Reciprocity failure correction table for TMax 100, TMax 400 and "Conventional film". I think you will agree with me there are few people on this globe who have done more rigorous testing than he did. You can find this table and others on his webpage. See the link to the PDF document of the "Reciprocity Compensation" table there.
Let's assume you have set your light meter set to 100 ISO, and get a measured or indicated time of 15 minutes, the maximum measured time Ralph included in his table. Look at the table, and see how Ralph indicates an adjusted time of 40 minutes for TMax 100 to get correct exposure.
Now a measured time of 15 minutes at 100 ISO, corresponds to a measured time of 4 minutes at an ISO 400 setting of the light meter. Looking up the adjusted time for TMax 400 for 4 minutes, results in an adjusted time of 14 minutes.
So, in exactly the same light circumstances, shooting the same scene, TMax 100 requires a 40 minute exposure, and TMax 400 just 14 minutes...
Even the "Conventional film" column, which I found quite accurate for HP5, still has a shorter exposure time listed at 30 minutes, than TMax 100. Of course, with measured exposure times rising above 15 minutes (not in Ralph's table), the conventional film (e.g. HP5) is likely to quickly loose out against TMax 100, but TMax 400 won't.
Looking at my negatives and the data provided by manufacturers, I have the feeling there are two big exceptions in the field of reciprocity: TMax 400 and Acros 100. Both excel at long exposures, loosing relatively little speed and needing the least reciprocity failure correction of all films in their speed range. In fact, as said above, Acros 100 is so good that it almost matches TMax 400 at long exposure times, although maybe more close to TXP 320 at exposures up to 2 hours, a film which did quite well too. Whatever the exact situation, using Ralph's adjusted time column for TMax 400 for Fuji Acros 100, should get you in the ballpark for a correct exposure for Acros 100 too.