Active S-boxes for AES with 8x8 MDS matrix One way to enhance the security of AES is by increasing the number of active S-boxes. Larger MDS matrices are used to increase the number of active S-boxes. Using a $4\times4$ MDS matrix results in 25 active S-boxes after 4 rounds.
How many S-boxes would be active after 4 rounds using an $8\times8$ MDS matrix?
It seems that using an 8x8 MDS will have 27 Active Sboxes after 4 Rounds(There may exist a trail shorter than 27). The image below shows the counting of active Sboxes. 


The Active and non-Active byte is shown as 1 and 0 respectively.
The 8 bytes of state which become input to the 8x8 MDS matrix are
colored in similar color (pink or blue).
The shift rows step shows new byte position in pink and white color.
The calculation is done under the assumption that 8x8 MDS matrix has
optimal branch number of 9 i.e change in one byte will change all
the 8 output bytes.  
If 8x8 Matrix similar to Camellia is used which
has branch number of 5, then count of active Sbox may be reduced.