While I certainly think that porting BG1 would be very nice, I'd also opt for a starting level as low as possible in IWG. 3rd edition class changing is much more flexible than 2nd edition dual- and multi-classing.
A character who for instance had 2 thief levels and then dual-classed to fighter didn't have much use, so it was not really missed that you couldn't create a thief(2)/fighter in BG2. For dual-classing, the starting level of BG2 was low enough.
This has changed in 3rd edition - giving your fighter character a thief level or two at some point has become meaningful but would be hampered by starting out with level 8 or so.
And, more important (at least imho): If you start at a high level, "multi-classed" characters won't be possible (that is, characters that simulate 2nd edition multiclassing by going up in two levels simultaneously). In IWD2, I had a bard/sorcerer who had equal levels in her classes for a long time; that wouldn't be possible if she had started as a level 8 bard.
Another point: While it may be that level 8 in 2nd edition and 3rd edition are roughly equal in power compared to the enemies, there are certainly differences in their charcter development until they reach that level. In 3rd edition, a lot of character development (feats, skills etc.) happens in the very first levels, while later on you just improve on what the character already knows - assigning second and third points. In 2nd edition, the first levels didn't have that importance and that influence for the development of the character.
Apart from that, I also think that the first levels are most fun.
So in conclusion I think that in this case, IWG should be viewed as a stand-alone game, and those gameplay issues of BG2-with-3rd-ed.-rules should have more weight than compatibility with a possible future BG1.
My 2 €-cents,
Cas