What we have here is a pretty clear cut of \"is this tactically soound, or is it just cheese\"
Cheese. for the un-initiated is ofter used to refer to tactics that are \"easy win\" something that anyone can do to win with little to no real effort with little to no real counter.
This \"500 block long weapon with power shields and engines\" well at this point in time resources are not much of a matter. which means such weapons easilly exist. This is one aspect of the balance that is yet to be really looked at, though many quality servers have decreased the loot ammounts to prevent players earning several million Cbills in under 5 mins.
As for being unable to defeat this weapon. well they are killable, most of the time by exploiting lack of turn speed.
However the following quote worried me alot.
I built a nice ship with 8 70 block long, 5x5 cannons. Had no power issues yet it was lacking in firepower against most other ships.
This is the carebear attitude that will kill this sort of game in the long run. not only is it mechaniclly unsound (5x5 cannon, you what? you only one 1 needs to be exposed right?) but it also shows that you believe someone who has more resources than you, spent more time getting those resources than you and generally has been \"working\" longer than you... should still be on your chosen level.
TL:DR; youre approaching the situation wrong. and youre also a carebear.