1) But in your case, cyclictest can generate 336000 (42*8000) hardware timer irqs per second VS 8000 from TDM card's.

Very interest results.

My example creates a lot of unnecessary interruptions

For VoIP only host more correctly measured so

cyclictest -t42 -p -d 80 0 -n -i -l 5000 6000

or

cyclictest -t42 -p -d 80 0 -n -i -l 10000 3000

If FS has one TDM cards then

cyclictest -t1 -p -d 80 0 -n -i -l 125 240000

If two TDM cards 

cyclictest -t2 -p -d 80 0 -n -i -l 125 240000

Where "-t" key equal number of TDM cards

2) According to Digium sources your TDM card have 20ms max buffer size(3ms default)

USB dongles have at least 20 ms.

Can you describe how you have found this information. It is very interest.