What diff ratios are you running? Nobody ever needs a 5 speed, it just makes freeway driving not so painful using 4.11's etc.
1st thing I'll say is what condition is your toploader shifter? They are well known to cause heaps of grief when broken. Is your shifter spring loaded to one side when in the neutral position? Can you feel the reverse gate when shifting into reverse? Can you slide straight into each gear?
Take your shifter off and put a recon kit thru it (from Rare Spares) and see if that makes a difference.
The slipping out of gear issue sounds internal.
The T5 and TKO are 2 separate gear boxes, both however are owned by Tremec. The T5 is the junior burger of them all. What are your driving habits, horsepower and intended applications for the car?
I don't agree that TKO's aren't good for drag racing. There are heaps of 9 and 10 sec cars in the US using them no probs. Only issue is difficulty shifting above 7000rpm (can be modded), but not too many people with that problem. Power shifting and missing a gear is suicidal too
Example: 1.3 60 footer and wheels hanging in the air...............10.1 @ 132