qgisorm.py

query = From(iface.activeLayer()) \
             .where('"Pit Type" = \'MH\'') \
             .limit(10) \
             .select("Pit Type", "SURVEY", "CATCHMENT", geometry=True)

query, layer = SQL("SELECT SURVEY, geometry FROM Pits GROUP BY Pit LIMIT 20", name="MyQuery")

query, layer = SQL("SELECT Pit, SURVEY, ST_UNION(ST_Buffer(geometry, 20)) as geometry FROM Pits GROUP BY Pit",
                   name="Buffers")

query.py

from qgis.core import QgsFeatureRequest
from PyQt4.QtCore import QUrl

class Query(object):
    def __init__(self, layer):
        self.rq = QgsFeatureRequest()
        self.layer = layer

    def where(self, expression):
        self.rq.setFilterExpression(expression)
        return self

    def in_(self, *ids):
        self.rq.setFilterFids(ids)
        return self

    def extent(self, rect):
        self.rq.setFilterRect(rect)
        return self

    def no_geometry(self):
        self.rq.setFlags(QgsFeatureRequest.NoGeometry)
        return self

    def limit(self, top):
        self.rq.setLimit(top)
        return self

    def select(self, *fields, **kwargs):
        if fields:
            self.rq.setSubsetOfAttributes(fields, self.layer.pendingFields())
        if "geometry" in kwargs and not kwargs['geometry']:
            self.no_geometry()

        return self.layer.getFeatures(self.rq)

def SQL(sql, name="querylayer"):
    sql = "?query={0}&geometry=geometry".format(sql)
    query = QUrl(sql).toEncoded()
    layer = QgsVectorLayer(str(query), name, "virtual")
    if not layer.dataProvider().error().isEmpty():
        raise Exception(layer.dataProvider().error().message())
    return From(layer).select(), layer

def From(layer):
    return Query(layer)

Gain readability and expressibility with an SQL like syntax.
Performances costs? (because ORM are now less use to fully take advantage of the underlying DB)