88-90 is about what they're at now, ish.
You sure? A 30% penalty would leave them somewhere at the 70 speed rating mark... and they feel like they're around there when I handle one. 15% would leave it at 84-ish. So it'd still be a boost.
Edit: It's probably 15% to damage and speed as it is, after playing with it for a bit on duel. So that'd put it at 83 speed rating and 30.6 cut. Which is a bit slow with my 21/15 1h/polearm build with 124 1h wpf... but could be used. 88 speed rating and 32 cut in 1h mode would however make it a reasonable weapon, and if they did it that way rather then just the penalty like now, the heirloom bonuses with it used in 1h mode wouldn't be wasted so much.
However, I'm not sure if it can have two alternate modes, it already has one (which is not very impressive, halfswording with it is slower then using it as a twohander, and I have 96 polearm wpf and 1 2h wpf, and weaker. If anything it seems stronger when used as a onehander then when halfsworded, no overhead, the swings are so weak and it's so damn slow to boot.
If I didn't have a masterwork SCS already I'd be tempted to heirloom it and make a hybrid geared for the longsword, but heirlooming yet another weapon... blah.