If your DM insists on using the (broken) EL system, you'll feel the hurt from even an EL +1 system before too many levels. So if EL+1 is your only option to play a vanilla tiefling, say no, doubly so for any type of caster.
Instead, ask him if all the EL +0 races can get a one-time gift of enough XP to bump them from 1st to 2nd level. Your tiefling will be a little behind at first, but will catch up relatively quickly, and won't suffer the overpenalizing from an EL +1.
While it is true that the race builder system has flaws and that you can easily min-max a race, keep in mind that the race builder was not meant for players to use. It's supposed to be a tool for GMs to create custom races.It is still inaccurate and while the GM has the final say the book is aimed at players.
That's incorrect. The book is aimed at both GMs and players and even says that the Race Builder is a GM tool for use by GMs or players with GM oversight.
I have no problem with Tieflings, Aasimar and Drow. I allow them as is, no LA. Drow Nobles I do LA -1 though.
SLA look a little flashy at the first glance but it's no big deal really, even at level 1. Even less of a big deal as the game goes forward.
Ask him if you can reskin an elf as a thiefling, using the elf rules but be a thiefling. If he want to run the game with some old rules, just be thankfull that he tells you in advance:)
Dont give up a feat or a level for what the thiefling gets, but if the GM have made a decision respekt him and it and move on.
And good luck.