With the open leaves, I'm changing my mind and leaning towards Acer palmatum. Here are some good descriptions from University of Oregon:
Acer shirasawanum
Broadleaf deciduous small tree or shrub, 10-30 ft (3-9 m) tall and similar width [in its habitat 50 ft (15 m)]. Leaves simple, opposite, about 6 x 6 cm, some larger, round (suborbicular), 9-13 lobed, each lobe has an acuminate tip, margin doubly serrate; petiole to 7 cm, glabrous (without hairs); fall color yellow. Flowers in small, erect clusters, petals white to pale yellow. Fruit in erect clusters above the leaves, glabrous, wings held near horizontal, to 2 cm.
Acer palmatum
Broadleaf deciduous tree or shrub, 15-25 ft (4.5-7.6 m), multi-trunked, variable forms, twigs glabrous, thin, bright red. Leaves simple, opposite, 5-13 cm long, 5-7(9) deeply cut lobes, lobes are lance- ovate to lance-oblong in shape, doubly serrate, fresh green above but color varies with cultivar, underside pubescent at the vein axils; fall color scarlet, orange or yellow. Flowers small, red to purple, appear along with new leaves. Fruit (samara) 1.3-2 cm long, spread at obtuse angle, much incurved, ripen in early fall.
Acer japonicum (I don't think this is it)
Broadleaf deciduous small tree or shrub, 30-40 ft (10-12 m) in good growing conditions; branches dark gray-brown and smooth. Leaves simple, opposite, 8-15 cm wide, roundish, 7-9(11) lobed, each cut to within halfway to the center (petiole), margins serrate. Flowers in large drooping clusters (corymbs), purplish red, appearing before leaves. Fruit in clusters hanging below the leaves, wings (samaras) about 3 cm long.
Good leaf description and flower descriptions - you should he able to decide which you have. Let me know as I'm very curious.
