I think its probably just that the old ones were so worn. I had mine changed on my vectra and the ride is so much firmer. I know it doesn't help you but this is probably the case.

Didn't Spax used to make adjustable shocks, i haven't a clue if they're still available, the current trend seems to be geared towards more or less solid suspension.

Delphi/Monroe would be on my shopping list as reasonable quality aftermarket.

Would have expected Toyota branded parts to be as good as any, but when the shocks were fitted some clot hasn't taken into their mind to shove an extra 6 psi in your tyres have they?

And Koni.

Try to get O.E. shocks, ask a main agent. My father's mk11 Cavalier (owned from new) was fitted with Monroe shocks at about 40,000 miles because he was wrongly told by an independant garage that the originals were too soft and worn out, the replacements destroyed the ride. When I took the car over I replaced all 4 with genuine Vauxhall parts and the ride was transformed. Previously I tried Monroe shocks on a Mk V11 Jaguar and I took them off after a 2 mile test drive.