I am not a big expert of SQL but just asking. Is it Possible the field ID to be like that:

|100        |
|  100      |
|        100|

If Not I think you have an Index on it & must Use it for optimization w/o any LEFTs & TRIMs.
Also When You replace the ID field you can make it always to be left justified
& then SELECT can be:

c_field = "100"+SPACE(LenOfTheIDField-3)
SELECT * FROM table1;
         where table1.id in (select table2.id from table2 where testfield=c_field);
union all;
SELECT * FROM table2;
         where testfield=c_field and;
               table2.id in (select table1.id from table1) order by 91,93 into cursor temp

Just a thought.