assume latency of x in ms ping
since castsequence "hangs" until the cast successfully finishes casting, we add +x ms (or x / 1000 in seconds) to the cast time of your spell

let's say your lightning bolt has a cast time of y seconds (2 seconds base / talented spell time minus whatever your benefits from haste are) and does z damage.

assuming a perfect world where everything happens automagically:
your dps for lightning bolt with a /cast is z / y
and your dps with the same spell with a /castsequence is z / ( y + x / 1000 )

your dps gain for casting lightning bolt with a /cast instead of a castsequence is then:
( z / y ) - ( z / ( y + x / 1000 ) )
if we do some fancy algebra, that turns into:
( z * x ) / ( 1000 * y ^2 + x * y )

which translates to:
( z - y ) / ( 1000 y ^ 2 ) dps gain per ms of ping

plug in random theoretical shaman that does average 10k lightning bolt damage per cast (including crits, lightning overload, whatever the hell you want) with a 1.6 second cast time:
3.906 dps gain per ms of ping

this means, if you are that theoretical shaman, you are losing about 1k dps on lightning bolt if you have 256 ping by using /castsequence lightning bolt instead of /cast lightning bolt . even if you have a good connection and have 25 ping, you're still losing around 100 dps

this equation of course becomes way more complicated when you add other spells, but i'm sure you get the point. using cast sequence is easier, but is costly even if you have great ping. bottom line is, if you have > 100 ms in ping and are wondering why your dps isn't cutting it, you should consider not using /castsequence at all.