There's more to it when you select a type and size MHG. The thickness MHG, brand of piston, and boost impacts compression pressure in the cylinders. You need to realize what pistons you choose and the machine work on the head/block does.
The stock piston size is 83mm...for each 0.020" oversize = 0.50mm. For example, a 0.040" oversize piston is an 84mm. The stock HG bore size is 84mm x 1.37mm thick
To select the thickness of a MHG (without increasing compression), you have to determine the amount of material removed from both the head and block. There is 0.040" for 1mm. If the machinist removes 0.005" of material from the head and 0.005" from the block, that is 0.010" total or 0.25mm. Since the stock Toyota HG is 1.37mm thick. Adding the above = 1.62mm thickness required to maintain stock compression on the cylinders.
The key to compression is to make sure you don't go lean under boost and cause a situation where detonation will occur. The thickness of the MHG and the compression ratio of the pistons both affect the static compression ratio. Here's the compression ratios for common pistons in the turbo 7M:
Stock = 8.4:1
JE = 8.4:1
Wisco = 9.1:1
Ross = 8.5:1
To figure your actual compression ratio, go here:
http://www.csgnetwork.com/compcalc.html
Stock piston Specs:
Deck Height: 0.020mm
Head volume: 40cc
Piston Volume: -19cc
Bore: 83mm (3.268")
Stroke: 91mm (3.583")
Rod Length: 152mm (5.984")
There are several different types of MHGs...take a look at this link:
http://www.supramania.com/forums/showpost.php?p=216396&postcount=5
The most common is the bead and the stopper made by HKS. The bead type is good for 90% of the engines out there. If you're running a monster motor, the HKS Stopper is the MHG of choice...it is the best, but is also the most expensive.
I'll touch on one other part of this puzzle...dynamic compression ratio. This takes into account when the intake valve closes and the piston actually begins compression of the air/fuel mixture. The ratio of the cylinder volume when the intake valve closes over the volume above the piston at TDC represents the dynamic compression ratio. This is what the air fuel mixture actually "sees" and is what "counts", not the static compression ratio. However, static compression ratio impacts the dynamic ratio and for the purpose of this discussion static ratio will get the point across. If you want to play with it here's a calculator:
http://www.rbracing-rsr.com/comprAdvHD.htm
What does this have to do with choice of MHG? As usual, it depends. On a stock set-up where you want to change over to a MHG, just figure out the thickness based on the amount of material removed from the head/block. You can use a Titan, Cometic, HKS or Greddy bead type and be fine...just get the one closest to the thickness to keep the stock compression ratio, follow the RA spec for decking the block/head, use ARP hardware torqued to spec, and remove rivets (if required) to avoid a bad seal. You do not want to use spray sealer on a new MHG.
On a more mid-range (400-500 HP motor), you want to consider how much boost you plan on and the pistons you use. You may want to use a thicker MHG to lower the compression ratio based on your plan. The bead type MHG's are fine for these plans. Fuel upgrades and the ability to control fuel start coming into play.
On a motor pushing the upper HP ranges, the HKS stopper is the MHG of choice...especially running 25-30 psi of boost. You need to know how the pistons you are using is affecting compression. A thicker MHG will likely be required to increase the volume of the combustion chamber.
You really can't say a MHG is the right (or wrong) choice until you know how much material is removed from the head/block. IMO, there's little difference in the manufacturer MHG quality...they will all work...go for the best price you can get for the MHG that suits your needs. I do know Cometic will make a MHG to your specs via special order...you can customize according to your needs and the RA spec required is a bit higher than a Greddy or HKS. In all cases, prep is the key and your HP plans for the motor.
