Richard,
The biggest problem I faced at these TL's was the lack of decent sensors and maneuver drive.
Active/passive EMS doesn't start untill TL A. At TL 8 the biggest active is a 2hex/60,000km Radar, and it's BIG. You can go for Folding HRT but High res thermal's not as good as Passive EMS and folding sensors arrays represent their own problems.
As for maneuver drive/sub light the best FF&S one at TL 8 is AZHRAE in Rocket mode. I've had difficultly using these in designs due to the fuel use.
I did design a 200Dt SDB at TL 9 if you're interested I could post it up. It has a 1g Fusion rocket for thrust and a fixed 2hex HRT but actually fights with two 5Dt remote vehicles also with fusion rockets and HRT but 2g and 1 hex each loaded with two missiles. I found this to be the best compromise as the armed Drones increase the sensor range of the SDB and are slightly faster.
I initially hoped it would be cheap and nasty, alas it turned out to be neither but still could be a threat to some civilian vessels if used correctly.
On the whole though I think TL 7 & 8 is just a bit too low. At these levels the respective governments would be better served (IMHO) by building small, solar powered, missile armed, and sensor equiped orbital defense platforms laser comm linked to dirtside defense centers equiped with MFD's.