he benefits of sequential injection are that:

* you may get slightly better mileage and lower emissions at low engine speeds,
* you can tune each cylinder's fuel amount independently (if you know how).

The effect on maximum horsepower is general negligible.

However, sequential injection does not necessarily mean you are injecting into an open intake valve all the time. The intake valve is only open less than 30% of the time in a typical 4 stroke engine. Once you are trying to produce more than about 25% of maximum HP your injectors are firing for longer than the intake valves are open. If your maximum HP is correctly calibrated to a safe 80% duty cycle, your injectors are injecting well over 50% of the time on closed valves. With MegaSquirt, fuel is injected on ignition events only, and while these are loosely related to cam events, it is a complex relationship, Try to 'squirt' through an open valve under all conditions is generally a bit hopeless, because:

* fuel that is injected when the valve is closed doesn't go anywhere, it just sits near the valve vaporizing until the next time the valve opens (some OEMs deliberately squirt against a closed valve to improve vaporization). So squirting against a closed valve does not generally affect the AFR for that cylinder (though there may be a small effect on the combustion quality, good or bad, depending on the port wall temperatures, etc.)
* the valve is generally effectively open (0.050") less 300° of a 720° 4-stroke cycle (and closer to 200 for 'stock' engines). So hitting the open valve requires precise cam related timing,
* to inject the full fuel charge at high loads/RPMs through an open valve requires very, very larger injectors, about 2.5 to 4 times larger than is usually recommended,
* as the duty cycle for the injectors rises, the injectors come closer and closer to squirting all the time, and injection timing becomes irrelevant.

However, for those applications for which sequential injection is desirable to maximize idle stability and smoothness, lower emissions, and make incremental improvements to cruise fuel economy, Bowling and Grippo are working on a 'router board' for MegaSquirt-II which WILL do sequential injection (and can be used to operate COP ignition systems as well). It will likely not be released until mid-2007.