You mainly have to be careful about the diameter of the magnet if going for deeper speakers, not just the actual depth. If the rear part of the speaker is larger diameter (existing ones are weedy) it fouls the heater pipe behind the speaker - I diddn't have to worry 'cos I have the hand vents blocked so just cut a chunk out of the pipe :devil:
This evening I fitted a pair of Hertz 100.3 10 cm speakers in there which even have a raised tweeter that wants to foul on the grille - you can get around that by putting a washer between the speaker & the grill where it screws on :slayer:
This shop has a decent range for comparison purposes.
http://caraudiosecurity.com/shop/product/products_id/8401.html
The curved grill is pretty easy to swap onto if the speakers are small enough, BUT it does mean that if the speakers are too deep you can't just easily cheat & have them slightly raised out of the dash like you can with some circular grilles - you have to put them behind it.