Less valvetrain weight is the "advantage" of running shim under bucket, also if done right it can't "spit a shim" out as the factory setup can. But in reality, the only way it's going to spit a shim with a shim over bucket (factory) setup is if it floats the valvs.

Less weight, thus: less HP to drive cams, less wear on cams, for the same spring stiffness, better valve control as less weight to accelerate. Can now rev higher since valvetrain has better control.

Downside is cost and time to do it.

FWIW, I run the double heavy springs, and have revved to 8,800 and not spit a shim. So if you're goal is only 7,400, I wouldn't say you "need" them, but less valvetrain weight is ALWAYS nice so if you want it, do it, but I'd run the heavy double springs before converting myself.

Going from SOB to SUB and titanium retainers saves right at .75lbs. Retainer savings is more than the lash caps weigh, so I'm guessing the ztech conversion wouldn't save quite that much without the retainer swap as well.

So I already have titanium retainers, but I believe steel keepers, so for me going to the ztech shim under bucket would be more like say, 0.2-0.3 lb savings? Sound right?

Ztech isn't SUB. No shims would be the point. You'd have to tip your valves or the bucket stem to get lash correct. So whatever the weight of your current shims is would be your savings.

Ok that makes sense. Sorry I didn't realize that, it's obvious I guess if you think about it... So considerable savings then, since shim is probably 30+% of the weight if I had to guess. Hmm....

@guttedmiata: Great point about feeler measurement. Certainly something to consider. I bought an extra shim on the thick side in case.

Good info guys. I've been convinced on it's merits. With new cams I will surely end up running SUBs or Zetec depending on base circle diam. of new cams.

thought I'd add my cost here for sub conversion:

~$200 for lifters from mazdacomp
$100 in shims.
$300 in labor for machining each shim. Of course I got a valve job, deck, etc from the machine shop for another $300, basically a whole rebuild top end rebuid.

Can I see pics of the machine you made? I'd love to be able to grind my own tips. Is there a temperature to keep them under, is there a hardened layer that only allows so much material cut ?