Pretty much everything TT posted whilst I was entering this... but it took so long I'm posting it anyway!
There's controversy about "high performance" coils. They tend to claim a higher spark voltage, but the spark voltage is set by the plugs - not the coils. Any appropriate coil will produce enough voltage to initiate the spark: the gas between the electrodes ionizes at some fairly fixed voltage depending on mixture and compression, then it begins conducting which creates the plasma arc. The coil voltage does not get above that ionization voltage, "extra" voltage in not used.
Some extreme performance engines with very high compression or exotic fuels have higher ionization voltages, some oddball plug geometries as well. This doesn't apply to our old bikes.
The usual choice for SOHC4 replacement coils is two Dyna DC1-1 or DC8-1 twin fire coils that fit on the stock coil mounting towers with a slight modification to the towers. Dyna coils come as 3 or 5 Ohm. The 5 Ohm coils (stock coils are 5 Ohm) are preferable in my opinion, the extra current draw from the 3 Ohm coils does not improve ignition from the installations I've seen, and they burn your stock points. I believe the HM ignition box can use 3 Ohm coils but I'm not sure - HM also sells ballast resistors to use 3 Ohm coils with normal points and these may be necessary with the ignition box. Standard points don't last long on 3 Ohm coils though, if the HM box fails and you switch it to bypass then the load is all back on the points. If you get Dyna coils you should pick up their coil sets as well, finding metal core wire and the coil terminals isn't as easy as many think.
You can also install dual fire coils from most 4 cylinder Hondas, some have replaceable cables unlike the old originals. Some of these also have different resistance, so check before you commit to them. Coils for electronic ignition models may be extremely low resistance (less than 1 Ohm).
The coils themselves don't often fail, it is usually the cables or caps that degrade.
Check cap resistance, the OEM resistance varies with model but will be 5kOhm or 10kOhm. The total resistance in the plug circuit should be what Honda supplied - you can use any combination of resistor caps and/or resistor plugs to achieve that. It's best to have all four caps and plugs the same for simplicity but electrically there's no difference between two 5k caps and one 10k cap plus one 0k cap.
It IS possible to replace the cables on a stock coil when the cables are old and ruined. It isn't pretty - but the coils are not visible, so who cares? You hack out the coil cases where each cable enters until you get to where they solder on and remove the cables. Then you solder in your new cables, and finally fill the case excavations with epoxy.
I don't think iridium plugs are worth the cost. From my research they are supposed to wear down less quickly, and I haven't seen my bike's plug electrodes eroding away. For car engines where the plug interval is tens of thousands of miles - and maybe the engine has to be lifted to get at them - OK.
