There is a higher SP multiplier bonus pre 20 (ranging from 4.0 to 1.8 as your level increases), which explains the dropoff you've been seeing.
Post 20 your best bet is alternating between behests and leve parties. With max links (one starter + three other players linking the same leve) and Guardian's Aspect, you are looking at 3.75 times the normal SP for each mob. On Dunesfolk for Dinner this can mean as much as 1k+ SP per mob, with 500-760 SP being more common for other leves.
You can keep soloing all the way to 50 if you want, but hooking up with an organized Linkshell with regularly scheduled leve runs is much, much faster. At the moment the six most popular leves for 30/40 seem to be:
Necrologos: Ranine Reveries (LV40 Broken Water)
Dunesfolk for Dinner (LV40 Broken Water)
Necrologos: Levinshower (LV30 Nophica's Wells)
Necrologos: The Moon's Mistress (LV30 Mun-Tuy Cellars)
Necrologos: Inferno (LV30 Cassiopeia)
Necrologos: Lightsome Verdure (LV30 Cedarwood)
Great Hoary Toads and Rockbound Mists are also good options once you hit the early 40s.
Edited, Jun 26th 2011 11:40pm by akirussan
Thanks Aiko. Rate up for actually taking the time and posting that. I hear people shouting for Dunesfolk all the time in Ul'dah.
I am ready for the changes to guildleves though. I don't think it is right that some leves offer a lot more SP than others. I guess this is why a lot of the population fail those leves on purpose so they can get them again next reset... We have to work with what we are given!
Bleam FFXI (Retired) 75MNK/SAM/NIN/SMN/THF
Bleam WoW (Retired) 80 Druid
Golf WoW (Retired) 80 Death Knight
Tewksberry WoW (Retired) 80 Hunter
Rahana WoW (Retired) 80 Priest
Purchase and instal a bigger hard drive, at the moment my PS3 has 500GB memory which is plenty.