The Storm Needler is a fantastic gun for the front center large mount of an Onslaught - I haven't found any other ship that I think its truly good on. The range is short, but as intrinsic_parity pointed out, range and mobility are linked and the Onslaught has a rather nice "press F to get in range" ability. And the payoff for the short range is very large: 750 Kinetic DPS on a single mount for 650 FPS with high accuracy just crushes shielded ships. The per shot damage of 75 isn't huge, but is sufficient (with skills) to rapidly cut through light armor and hulls. It is rare for any frigate or destroyer that is caught in the arc to survive for more than a few seconds.
The Storm Needler, Burn Drive, and the Onslaughts toughness give it an inverted range profile compared to many ships. Burn drive gets the Onslaught into close range on all but the fastest ships without having to "flux duel" their way in. It is effective solo but the ship might take a few hard shots; if you have even a single ally that has caught the enemy's attention (and guns) you can burn drive in without any fear. Once in close range the other ship's shields will be rapidly destroyed by the Storm Needler - as it tries to flee it stays in the range of armor and hull killer guns even once outside the kinetic barrage (can't take TPC's on shields when your flux pool is maxed).
You do need to build with it in mind and watch the flux output of the ship - I often play around extensively with my Onslaught weapon groups in order to manage firepower (generally TPC's in one group as siege/hull killer weapons, storm needler and maybe a few forwards in another, and then side guns in another, and finally Flaks + 2 LAGs). That said, the Storm Needler actually has very good flux "handling". It is a smooth flux buildup with no sudden burst or dump - it won't surprise you into an overload like lances will.
Re: Range vs Speed burst
What a speed burst gives you that range doesn't is the ability to a) ambush ships and b) rapidly re-position to take advantage of an opportunity. Basically any time your allied ships have the attention of an enemy force and you aren't about to get a kill on your current enemies, with a speed burst you can turn away from your current fight, blast in, and absolutely murder the enemy ships that already have high flux. With a Paragon you just can't do that - yes you have a supremely powerful mobile battle station, but you cannot dynamically react to opportunities in combat.
This is somewhat an argument for the Conquest as well as the Onslaught, but I think the Onslaught does it better because of its toughness. A Conquest needs to always be worrying about strike fighters and flanking destroyers because of its poor defense, so it can't always take advantage of rough situations. It needs to pick only the best opportunities. Onslaughts you can just burn straight into an enemy formation of cruisers and physically ram them apart, and usually the ships just won't have the firepower to seriously damage the Onslaught before they are fluxed out and destroyed.