RT/DW format as compared to Black Crusade? What does that mean? Don't they all use the same layout?
I would maybe boost the damage over 2d10. Something like 2d10+2 or maybe 5.
Not quite. Black Crusade liked to tone back the dice rolled, in favor of keeping it simpler with more use of Proven (X). For example, a RT heavy bolter is 10 shots, 2d10+2 X, Storm, Tearing, while the BC version is 6 shots, 1d10+8 E, Tearing. Lascannons lose a whole die of damage from their DW variant, and gain Proven (3), and while one might say "yeah, but the DW one is an Astartes Lascannon", so is the Chaos Marine's lascannon, but BC decided to step back from "every Space Marine weapon is inherently better" principle.
Hmm .. Comparing it to the other profiles in 40k TT and their counterparts in the IH i'd go with
90m -/-/20 2d10 I Pen 2 Clip -- Rld 2 Full Storm
The Storm Quality to better represent the tight hail of bullets. But if that makes it too strong leave it out.
Personally, I'd go with 1d10+12 I, Pen 0, Storm - only rolling one die for damage increases the minimum damage per hit and means you can roll for damage for all the hits at once (a time-saver when you're scoring lots of hits). I'd also assume a clip of 2000, and a Reload value of 10+ full actions (it's a vehicle weapon with a huge ammo feed), but those are more personal stylistic preferences.
Full Auto 20 basically wastes shots, it should be noted. Beyond -/-/13 and an exceptionally skilled marksman in perfect conditions (BS70+, +60 from circumstances), shots will be wasted because nobody can score sufficient degrees of success. With an assumed BS40 gunner, at short range with full-auto and a half action aim (50% chance to hit), you'll only score 5 DoS maximum, which equates to 10 hits with Storm. If that's a deliberate choice, that's one thing… but it's something to consider if it wasn't a deliberate matter.
I might also (or instead) be inclined to add a special rule:
"Torrent of Fire: Sustained fire from the weapon is horrific to behold, and few things can stand firm before such an onslaught. Instead of making a normal Full-Auto attack with the weapon, you nominate a single point within range - this is the centre of the Torrent of Fire. Make the Ballistic Skill Test as normal, with a -20 penalty instead of the normal -10 for Full Auto. Any creature within a number of metres equal to the Degrees of Success on the Ballistic Skill Test of the centre of the Torrent of Fire is hit a number of times equal to the firer's Ballistic Skill Bonus, and may attempt to Dodge as normal. In addition, when attempting a Suppressive Fire action with this weapon, it imposes an additional -20 penalty on Pinning Tests."
As a deliberate choice, I'll say I was deliberately trying to emulate the TT weapon, but that game doesn't penalize for multiple shots; certainly, the BS 3 IG are likely to miss with a few, but with 20 shots, that's okay. Killing Orks is about pouring as many bullets into their mobs as possible. If that isn't likely to work here, I'll have to give it some thought. Other stuff has changed the RoF of the same weapon (usually lascannons and heavy bolters), so I probably could here, too. Assuming 4 degrees of success on average, taking into account BS 40, full auto burst +20, full aim +20, auto-brace from the heavy carriage, perhaps a RoF of 10, 20 with Storm, will work better. semi and Full Auto get bonuses to hit, not penalties, at the expense of ammo.