I'm using HS105 since almost 9 years... The secret to let fly the HS105 at high speed is to use ridiculously small fins for her size. I use a 32 cm fin with 8.2 sail or a 26 cm fin with 6.6 & 5.4 sails when well- to over-powered.
Big fins are usually used with large sails when underpowered to well-powered: a 32 cm fin with 6.6 m2, a 40 cm fin with 8.2 m2 or a 55 cm fin with 11 m2 sail. Sure, other boards are faster than HS105 but no current iSonic can match such a huge wind range with just 4 fins and 4 sails. HS105 is a true slalom M-XL-mini Formula crossover that is still unique and remains unmatched so far. A true collector for those who wants a single racing hull instead of 3.