Many model boats like scale yachts, fun- and race-boats, industrial, scientific and military vessels with a length between 50 and 150 cm and a weight between 1,5 and a maximum of 4 Kg per installed KMB JET28 drive usually already reach descent "scale fun speeds" at relatively low motor rpms, respectively, when trimmed to 20000 rpm, likely reach the top speed possible with the KMB JET28(s) and the corresponding hull (often already above the "reasonable" speed limit the hull was constructed for).
Many scale models already reach the "feeling comfortable zone" with setups for 8000 to 15000 rpm.
A setup for maximum thrust requires the setup for 20000 rpm (allowed maximum rotational speed for the KMB JET28) and roughly 800 Watts power for single drive setups (boat with only one KMB JET28 installed). Dual / Multi setups raise the efficiency for each drive. Speeds up to 45 kmh are possible if the hull supports such speeds. It strongly depends on the running capabilities of the hull what top speeds are possible.
The rule of thumb "The lighter, the faster" is not valid for jetboats. If you build it too light, it will uncontrollably and untrimmably "hop around the lake". If you build it too heavy, ... it will capsize. The thruth for a well running jet boat model lies somewhere in between ... A jetdrive needs water to breathe.
Happy boating and modelling!