My Landenberger spi has been measured on Texel, these are the measurements:

SF 3,40 (foot)
SL1 8,06 (luff)
SL2 6,7 (leech)
SMG 2,6 (mid girth)
area spinnaker 16,97
SMG as % of SF 76,47 (mid girth)

A picture: http://www.catamaranparts.nl/BladeF16b.htm
I think the spi is fairly easy to handle, doesn't collapse too easy. Needs a fairly high luff tension otherwise it will become very hard to prevent collapsing.

Maybe a little better spi than my last Goodall (2004) but way better than my first Gooddall (1999)

Geert