You could override that and any other model methods like this:

class task(osv.osv):
    _name = 'project.task'
    _inherit = 'project.task'

    def _hours_get(self, cr, uid, ids, field_names, args, context=None):
        res = {}
        cr.execute("SELECT task_id, COALESCE(SUM(hours),0) FROM project_task_work WHERE task_id IN %s GROUP BY task_id",(tuple(ids),))
        hours = dict(cr.fetchall())
        for task in self.browse(cr, uid, ids, context=context):
            res[task.id] = {'effective_hours': hours.get(task.id, 0.0), 'total_hours': (task.remaining_hours or 0.0) + hours.get(task.id, 0.0)}
            res[task.id]['delay_hours'] = res[task.id]['total_hours'] - task.planned_hours
            res[task.id]['progress'] = 0.0
            if (task.remaining_hours + hours.get(task.id, 0.0)):
                res[task.id]['progress'] = round(min(100.0 * hours.get(task.id, 0.0) / res[task.id]['total_hours'], 99.99),2)
            # TDE CHECK: if task.state in ('done','cancelled'):
            if task.stage_id and task.stage_id.fold:
                res[task.id]['progress'] = 100.0
        return res

The think in this case is that the _hours_get method is defined as the function to be called in many functions methods and due to the old api define fields function's compute function by reference yo need to redefine the fields that use the function and the others functions by reference they use, like _get_task in the next code example. At firsts they are:

effective_hours, total_hours, progress, delay_hours

So you need to also do this:

    def _get_task(self, cr, uid, ids, context=None):
        result = {}
        for work in self.pool.get('project.task.work').browse(cr, uid, ids, context=context):
            if work.task_id: result[work.task_id.id] = True
        return result.keys()

    _columns = {
        'effective_hours': fields.function(_hours_get, string='Hours Spent', multi='hours', help="Computed using the sum of the task work done.",
            store = {
                'project.task': (lambda self, cr, uid, ids, c={}: ids, ['work_ids', 'remaining_hours', 'planned_hours'], 10),
                'project.task.work': (_get_task, ['hours'], 10),
            }), 
        'total_hours': fields.function(_hours_get, string='Total', multi='hours', help="Computed as: Time Spent + Remaining Time.",
            store = {
                'project.task': (lambda self, cr, uid, ids, c={}: ids, ['work_ids', 'remaining_hours', 'planned_hours'], 10),
                'project.task.work': (_get_task, ['hours'], 10),
            }),
        'progress': fields.function(_hours_get, string='Working Time Progress (%)', multi='hours', group_operator="avg", help="If the task has a progress of 99.99% you should close the task if it's finished or reevaluate the time",
            store = {
                'project.task': (lambda self, cr, uid, ids, c={}: ids, ['work_ids', 'remaining_hours', 'planned_hours', 'state', 'stage_id'], 10),
                'project.task.work': (_get_task, ['hours'], 10),
            }),
        'delay_hours': fields.function(_hours_get, string='Delay Hours', multi='hours', help="Computed as difference between planned hours by the project manager and the total hours of the task.",
            store = {
                'project.task': (lambda self, cr, uid, ids, c={}: ids, ['work_ids', 'remaining_hours', 'planned_hours'], 10),
                'project.task.work': (_get_task, ['hours'], 10),
            }),
        
}

Doing those steps you will be ok to override that and others functions. In Odoo 8 functions fields have changed and now the functions that compute the values are not passed by reference so you just need to override the defined function and you are done