I can't say I've spent too much time on those races, but there's few other cars available in-game with such a high power-to-low weight ratio, and have a dirt tire option. I've tried the FTO LM and del Sol LM, and even those two can't keep up with the Escudo; but I can't say I tried more than 5-6 times, each. Some of the factory cars can get more power with a Stage 4 turbo, but will have too much lag to be useful with all the switchbacks and hairpins. Might be enough for some of the other dirt/mixed surface races, although I've won many of them with tuned Imprezas, Evos, and even that FC RX-7. Never really tried the GT40 on dirt, might be interesting. I don't think it has the overall muscle to compete with the Suzuki twins.
Better get a Cultus for those two difficult uphill Pikes Peak races...it's not an impossible challenge, but something different. If you really dislike the rally races, just get an Escudo.