SQL 2005 RowCount Paging Stored Procedure

How would I make the following SQL 2005 code into an efficient stored procedure:

WITH TestRows AS
(
SELECT *, ROW_NUMBER() OVER(ORDER BY ID) AS 'RowNum' FROM TEST %s
)
SELECT * FROM TestRows WHERE RowNum BETWEEN %d AND %d

%s is replaced by a where clause defined at runtime. E.g. %s = WHERE ID = 1 or %s = "WHERE User = 'bob' AND Result='Pass'"
%d is replaced by numbers defining the rows to turn, e.g. 0 and 499 or 500 and 999 respectively

I did the following which works

PROCEDURE [dbo].[sp_TestPage]
                -- Add the parameters for the stored procedure here
                @Where varchar(255),
                @low varchar,
                @high varchar
AS
BEGIN
DECLARE @sql varchar(1000)
                -- SET NOCOUNT ON added to prevent extra result sets from
                -- interfering with SELECT statements.
                SET NOCOUNT ON;

    -- Insert statements for procedure here
                set @sql = 'WITH TestRows AS ( SELECT *, ROW_NUMBER() OVER(ORDER BY ID) AS RowNum FROM TEST ' + @Where + ')
SELECT * FROM TestRows WHERE RowNum BETWEEN ' + @low + ' AND ' + @high
exec(@sql)
END

But as I am effectively creating the sql dynamically in the stored procedure (as the WHERE clause is NEVER known at until runtime) it actually runs marginally slower than sending over the raw sql!

Many Thanks,