Experimente assim:
EDIT:mudei a abordagem para lidar com valores não numéricos como '123abc' também.
declare @ids table(idList varchar(100))
insert into @ids values
('1.1')
,('1.2')
,('2.8')
,('2.7')
,('6.5')
,('6.5.1')
,('6.5.15')
,('7.1')
,('8');
select idList,padded.OrderBy
from @ids as ids
cross apply(select cast('<r>' + replace(idList,'.','</r><r>') + '</r>' as xml)) as AsXml(val)
cross apply
(
select right(' ' + rtrim(x.y.value('.','varchar(max)')),10)
from AsXml.val.nodes('/r') as x(y)
for xml path('')
) as padded(OrderBy)
order by padded.OrderBy
