You don't have to go that far back to find a time when Hispanic immigration was predicted to be a boon for the Republican party.  When the GOP was really pushing "family values" in the 80s and early 90s, having a growing population which was more observantly religious and more committed to the traditional ideas of family was seen as benefiting Republicans.  It wasn't until the party starting pushing anti-immigrant policies that the tide began to change.  

The best way to not create 800,000 new Democratic voters would probably be to create 800,000 new GOP voters.