The length difference is not that big of a problem, although 150 meters is slightly above my threshold where I consider re-doing the whole layout again (it's around 100 meters for me depending on how hard it is to nail the actual layout, how long the track is, etc.; this value changes from track-maker to track-maker).
However, you do have some inaccuracies in your layout - and trust me, you want these corrected sooner rather than later when you are already building trackside landscape and objects for the circuit. I've learnt this the hard way.
For example take a look at the last corner on Google Maps - you can see that it's a single apex turn rather than the double apex one you drew. The previous turn, the 180-degree one, has an earlier apex than what is in your .dat - it's hard to describe with words, but the corner turns 'more' in its early stage and its radius begins to increase earlier. Finally, Turn 3 seems to have a larger radius.
I don't know if you have followed the trace of your baseline image - the pic you posted on the 26th seems to have the correct Turn 3 and last turn layout, so I would consider a revisit there. However, the 180-degree turn seems to be a touch off on that one as well. If you are not satisfied, consider mapping the track in Google Maps and then insert that picture into GP4 Builder to base your track on. F1 Virtual wrote an excellent tutorial on how to do that (search 'f1 virtual inserting google earth picture' on Google).
Plus, a general tip: if you build elevations before you're done with things you can read from your baseline image (layout, track width, kerbs, bank widths, verges and pitlane layout), you are essentially wasting your precious time, because then when you want to proceed to the remaining items from the above list, you won't be able to place them on their proper spots. So, the tip would be: insert the baseline image, draw the initial layout, align the track width where needed (in the case of Barbagallo, maybe in Turn 4 first and foremost), place the kerbs, create the properly-sized run-off areas (I hate that part, haha, I find it way too time-consuming :D), and do the pitlane properly. After that, you can start building elevations which would then raise and drop your .dat from the surface of your baseline image. Also, your previously created parts, kerbs, rail alignments might provide help when you come to nail your elevations.
I know this sounds a lot, and I won't lie, creating a 95-99% spot on .dat does take time - however, you do have a lot of time, nobody is hurrying you (politely ignore the 'IZ IT RELEASED YET??!' type of comments) you do it when you want to do it.
My workthread - [www.grandprixgames.org]
Full of classic F1/non-F1 track layouts
My blog about F1 performance analysis - [thef1formbook.wordpress.com]